Iron Metabolism in Detail

I have recently been diagnosed with iron deficiency anaemia and am having tests to try to identify a cause and rule out serious pathology.

Iron metabolism is extremely complicated, but also very interesting. The human body has evolved to utilise iron as a means of carrying oxygen to where it is required. Iron is potentially a toxic substance and the absorption is controlled in a clever and finely balanced way by the intestines.

What follows is a detailed article about iron metabolism.

Haemostasis, transport and the storage of iron in the human body

As a component of metalloenzymes and complexed to form haem, iron participates in the transport of oxygen by haemoglobin and myoglobin and in the harnessing of metabolic energy by cytochromes of the electron transport chain.

Iron is abundant in the environment and is a common element in the Earth’s crust, but its electrochemistry poses exceptional problems for living organisms. The metal exists in two readily interconvertible redox states (divalent ferrous iron, Fe2+, and trivalent ferric iron, Fe3+) which are highly reactive. In the environment, most iron is oxidized to the trivalent state which, under neutral conditions, is then rapidly hydrolysed to insoluble polyhydroxide complexes which cannot be assimilated. High-affinity iron-binding proteins, which form stable ferric complexes, have evolved to facilitate iron transport and delivery to sites of storage and utilization, including haem biosynthesis. Free iron promotes formation of damaging oxygen free radicals, which mediate the injury to cells and tissues that characterizes iron storage disease. Iron is highly electroreactive, and readily catalyses formation of hydroxyl radicals as a result of interactions between superoxide and ferric ions. Tissues with significant iron storage show peroxidative injury in membrane lipid fractions; and, whatever their physiochemical basis, common mechanisms of iron toxicity clearly exist, since the pathological and clinical manifestations of all iron-storage syndromes, including secondary haemochromatosis associated with blood transfusion and the iron-loading anaemias, are almost identical. In disorders of iron overload the iron-binding capacity of plasma transferrin may be exceeded so that a proportion of the iron present in the blood remains reactive as a low-molecular-weight species only loosely attached to plasma proteins. Nontransferrin iron in human plasma stimulates the peroxidation of unsaturated lipids and can form reactive complexes that damage DNA—thus suggesting a mechanism for genome toxicity and carcinogenesis related to iron overload.

In humans, iron deficiency is probably the most frequent organic illness worldwide. It affects infants, children, young adults, and older people in many populations. Iron deficiency is associated with anaemia and nonhaematopoietic disturbances that impair work efficiency and contribute to chronic ill health as well as loss of mucosal integrity; iron-deficiency anaemia is frequently associated with pica, which has important environmental and behavioural associations with hookworm infection. In any event, the prevalence of iron deficiency provides strong evidence of the critical availability of iron as a key nutrient. The harmful effects of iron result directly from its electrochemical properties and potential to generate reactive oxygen and nitrogen species which injure cell structures—including DNA. There are many causes of excess iron in the tissues, but all represent disturbances of iron homeostasis that overwhelm the mechanisms which the body uses to acquire, transport, and store iron safely.

Body iron composition

The total amount of iron in the adult body is between 3 and 4 g, most of which is coordinated in protoporphyrin IX as haem. Haem is found principally as haemoglobin and myoglobin, although appreciable quantities are found in the viscera, especially the liver, kidney, and intestine. Cytochromes of the electron transport chain and of the P450 system for the metabolism of xenobiotics are abundant in these organs and remarkably selective regions of the brain. In an adult, about 2.5 g of iron is complexed in haemoglobin with an additional 0.5 g as myoglobin in the muscles. In the plasma compartment, very small amounts of iron circulate, bound in the ferric form to the glycoprotein transferrin—this protein is normally only one-third saturated with iron, so that with a mean concentration of 3 g/litre for a protein of molecular weight 80 000, it represents less than 2 mg of elemental iron. The normal concentration of ferritin in the serum does not exceed about 250µg/litre; and like transferrin, this does not itself contain appreciable iron—nonetheless, serum ferritin faithfully reflects the stores of iron in the body. Iron is stored in the mononuclear phagocyte system (previously known as the reticuloendothelial system) principally as intracellular ferritin and its proteolytic degradation product, haemosiderin. Body iron stores do not exceed 1.5 g in men and are usually 0.5 g or less in adult women. Nonhaem deposits of iron that serve as stores in the iron-rich tissues may be visualized by staining with Perls’s reagent (acid potassium ferrocyanide) with which they give a strong Prussian blue reaction. Faint staining with Perls’s reagent may be observed in normal parenchymal liver cells, but in health the principal deposits of storage iron are observed in bone marrow and spleen macrophages as well as in Kupffer cells of the liver.

Erythropoiesis and iron balance

The mean lifespan of the red cell is 120 days and thus approximately 1% of the steady-state haemoglobin pool is turned over daily—this requires de novo synthesis of approximately 6 g of haemoglobin into which 20 mg of iron is incorporated. The principal fraction of the iron required for daily haemoglobin production in the basal state is recycled from senescent red cells after their destruction by macrophages; the iron is delivered to the erythron in the plasma by transferrin that binds to cell surface transferrin receptors on erythroid precursors. The transferrin–receptor complex is internalized and, after acidification in endosomes, the iron is released leaving the apotransferrin to be recycled to the surface and reutilized. Transferrin is the principal mediator of iron delivery and transport about the body.

Under circumstances in which erythropoiesis is stimulated, e.g. under conditions of reduced oxygen saturation, after bleeding and haemolysis, as well as in dyserythropoietic conditions (including thalassaemia and megaloblastic anaemia), uptake and delivery of iron are greatly increased. Increased delivery of iron occurs in association with an expansion in the number of erythroid precursors that express cell surface transferrin receptors under the influence of the hepatorenal hormone, erythropoietin.

Iron homeostasis

Iron, an essential nutrient, is fastidiously conserved by the body and only a fraction of that which is utilized in the bone marrow is subject to obligatory daily losses through the exfoliation of epithelia and intercurrent blood loss, such as that incurred in trauma or menstruation. These requirements are met from the diet by the specific absorption of iron in the upper small intestine. The amount of iron available in the diet varies greatly, and even under optimal circumstances only a fraction is normally absorbed: in adult men the daily requirement is on average 0.8 mg, whereas in adult women of the reproductive age group, the requirement is usually more than 2 mg daily—the recommended daily allowance in the diet is 10 to 20 mg depending on the bioavailability of food iron components. Inorganic and haem iron complexes are released by digestion; there is a belief that haem iron may be more readily absorbed than inorganic iron in the human intestine and, depending principally on the content in meat, may constitute an important source of iron in nonvegetarians. Dietary phytates and medication including antacids and tetracycline antibiotics, as well as proton pump inhibitors, H2 antagonists, and prior upper gastrointestinal surgery, strongly influence the intraluminal bioavailability and hence absorption of food iron. The requirement for iron is greater in patients with recurrent bleeding, or in those who are blood donors; iron requirements are also increased during periods of growth in childhood and adolescence. In pregnancy, the daily requirement may be as much as 5 mg and the maternal investment of iron, depending in part on peripartum blood losses, may be as much as 1.5 g—this greatly exceeds the savings due to the cessation of menstruation. Given its iron content, the exsanguination of 1 ml of blood constitutes a loss of approximately 0.5 mg of iron; this relationship facilitates estimates of iron requirements as a result of blood losses, e.g. those incurred by menorrhagia (>80 ml/month) or from other sources.

Iron absorption

In health, iron absorption in the duodenum and upper jejunum is a finely regulated process which matches the acquisition of iron from the diet to body requirements for erythropoiesis and to replace obligatory losses. Genetic studies of mutant strains of mice with abnormalities of iron metabolism have shed light on the iron-absorption mechanism. The divalent metal transporter protein, DMT 1, which is expressed in the upper small intestine and cells of the erythron, is essential for uptake of ferrous ions. The human DMT1 gene maps to the long arm of chromosome 12 and encodes a 12 membrane-spanning protein that is expressed in the apical membrane of the upper intestine. DMT 1 is also produced in developing erythroid cells, in which it is responsible for the intracellular delivery of iron derived from transferrin for haemoglobin synthesis. Contemporaneous studies in experimental animals have identified a ferrireductase localized also to the intestinal brush-border membrane. Expression of mucosal ferrireductase is specific to the apical microvillous membrane of mammalian intestinal mucosa and appears to be induced in response to nutritional iron deficiency. Dietary iron is often present as a complex in haem—a component of haemoglobin, myoglobin, and tissue cytochromes. Haem iron occurs principally, but not exclusively in meat, which represents an important facultative source of iron in the diet of many humans. The uptake of haem by enterocytes is probably mediated principally by a membrane protein, but the molecular identity of this putative carrier entity has yet to be clarified. Egress of ferric ions from macrophages and enterocytes is mediated by a protein, ferroportin, which is localized to the basolateral membrane of intestinal epithelial cells; heterozygous deficiency of ferroportin gives rise to excess storage of iron in the macrophages. A putative copper protein, hephaestin, which maps to the X chromosome and has sequence similarity to caeruloplasmin, apparently mediates oxidation of ferrous iron and cooperates functionally with ferroportin in promoting the transepithelial transport of iron in enterocytes.

Under conditions of iron deficiency or on depletion of body iron stores, a greater proportion of the bioavailable iron is assimilated by the intestine. Hypoxia similarly increases the absorptive capacity of the small intestine. For reasons that are not fully understood, certain anaemias, particularly those associated with ineffective erythropoiesis and dyserythropoiesis, are also associated with enhanced absorption of iron in the intestine. Where the anaemia is long-standing, e.g. congenital or acquired sideroblastic anaemia, or in haemoglobinopathies such as β thalassaemia, inappropriate intestinal absorption of iron which accompanies massive expansion of the erythropoietic marrow, may be such as to cause iron overload leading to tissue injury—secondary haemochromatosis—even in the absence of iron loading from multiple transfusions of red cells.

The regulation of iron balance by the intestine normally protects the body from iron-rich diets; only under exceptional circumstances, such as the ingestion of alcoholic beverages containing abundant iron as a result of toxic manufacturing processes (e.g. the kaffir beers that are fermented in iron pots by the South African Bantu), does excess dietary iron lead to iron storage disease. It seems probable that those individuals who develop iron storage disease because of long-standing excessive ingestion of highly available iron, do so as a result of the operation of genetic cofactors such as mutant alleles of the adult haemochromatosis gene product, HFE, or because of an underlying haematological disorder such as α- or β thalassaemia trait.

Hepcidin, a member of a family of cysteine-rich peptides with antimicrobial activities, is an ‘iron hormone’ which regulates iron metabolism by inhibiting efflux of iron from macrophages and enterocytes. Deficiency of hepcidin is associated with iron overload, while excess release of hepcidin inhibits the release of iron from the storage compartment and reduces the net absorption of iron. One action of hepcidin appears to be mediated by binding to the ferric ion transporter ferroportin; hepcidin induces the endocytosis and intracellular breakdown of ferroportin and thus diminishes efflux of iron from enterocytes and macrophages into the bloodstream. In hepcidin-deficient humans (with juvenile haemochromatosis) and mice, intestinal absorption of iron proceeds in an unrestrained manner and marked storage excess with parenchymal injury results. Latterly it has been shown that bone morphogenetic protein (BMP) signalling induces hepcidin expression as a result of activation of serine/threonine kinases present on the surface of liver parenchymal cells: this effect is apparently mediated by phosphorylation of several of a group of intracellular transducing molecules (receptor-regulated or R-Smads), which then form heteromeric molecular complexes with the mediator Smad 4. After translocation to the cell nucleus, R-Smad—Smad 4 complexes appear to promote transcription of specific genes including hepcidin. The large multidomain molecule haemojuvelin—mutations in the gene for which, as with hepcidin, induce early-onset or juvenile haemochromatosis in humans—appears also to be a coreceptor for bone morphogenetic protein signalling in the regulation of hepcidin expression. At the time of writing however, all the molecules that interact with haemojuvelin in this pathway, including the range of bone morphogenetic protein ligands, have yet to be identified.

Recently several members of a Sardinian family have been reported with microcytic anaemia due to defective iron absorption and utilization: iron-deficiency anaemia unresponsive to oral iron and only partially responsive to parenteral iron administration was inherited as a recessive trait. After excluding the involvement of known genes implicated in iron metabolism, a genome-wide search identified a locus encompassing the matriptase-2 gene TMPRSS6 (also known as transmembrane protease, serine 6), which was shown to harbour a homozygous splicing mutation, predicted partially to inactivate protease function. Plasma and urinary hepcidin concentrations were later shown to be inappropriately elevated. The corresponding murine gene (Tmprss6) has been shown to be an essential component of a pathway that is sensitive to iron lack and suppresses the release of hepcidin. Finally, there is considerable research activity in the field of iron sensing; although little is known about the means by which cells detect hypoxia and iron requirements, complex molecular interactions between the HFE protein (mutations in which predispose to adult haemochromatosis) and transferrin–transferrin receptors 1 and 2 have been strongly implicated in this process. At the time of writing, however, these putative interactions are based largely on inferences from structural analysis of soluble protein complexes in vitro and confocal microscopy.

The evaluation of body iron status

Clinical features

The most useful clinical measures of iron status include the detection of pallor and nonerythropoietic manifestations of disease including angular cheilosis, atrophic glossitis, and dystrophy of the nails with longitudinal ridging and koilionychia; moderate hair loss may also be a feature of integumental iron deficiency. Iron deficiency induces behavioural changes in experimental animals. In humans, unusual syndromes of food craving (pica) have long been recognized and usually respond to iron supplementation: this includes craving for soils and the ingestion of silica-rich earths as a cult practice in black populations of the southern United States of America—geophagia. Pagophagia (ice-craving) combined with the abnormal taste preferences of pregnancy may account for the bizarre food craving that constitutes part of the folklore of pregnancy. Severe iron deficiency may occasionally be associated with splenomegaly and the signs of underlying disease include peripheral oedema (hypoalbuminaemia associated with massive hookworm infection) and oronasal and palatal telangiectasia associated with Osler–Rendu–Weber disease (hereditary haemorrhagic telangiectasia).

Laboratory findings

The most useful measurements, apart from those identifying the hypochromic microcytic anaemia accompanied by abnormal blood cell indices and confirmed by microscopy of the blood film, involve surrogate measures of body iron stores. A raised platelet count suggests a haemorrhagic cause of the iron-deficiency anaemia. Iron-deficient erythropoiesis is associated with failure to utilize free protoporphyrin in red cell precursors and thus a raised free erythrocyte protoporphyrin concentration in the peripheral blood; this may be easily identified by the use of portable fluorimeters using only a few microlitres of blood drawn, for example, as part of population screening.

In iron-deficiency anaemia, the absolute concentration of transferrin is raised: there is an increase in transferrin iron-binding capacity (TIBC), which is reflected by a decrease in serum iron and serum iron transferrin saturation. Such measurements may often serve to discriminate the hypochromic microcytic anaemias of thalassaemia and sideroblastic anaemia from true iron-deficiency anaemia. Such discrimination is vital before a commitment to treatment by iron supplementation is undertaken.

Measurement of the serum ferritin is often helpful in iron deficiency; serum ferritin concentrations are low, reflecting reduced or absent body iron stores. Neither the serum transferrin saturation nor serum ferritin, however, are absolutely infallible measures of iron deficiency: serum transferrin iron saturation may be artificially elevated with a low transferrin and low serum iron in chronic inflammatory states associated with the anaemias of chronic disorders. Likewise, serum ferritin serves as an acute-phase reactant and may be elevated in malignant disease (especially lymphomas including Hodgkin’s disease), or released from the liver in hepatitis and in chronic inflammatory states. Recently there are advocates for the measurement of free circulating transferrin receptors which may be determined by immunoassay. Expression of soluble transferrin receptor protein is enhanced under conditions of iron deficiency and plasma concentrations are elevated in the presence of functionally iron-deficient erythropoiesis; however, greatly increased serum transferrin receptor concentrations are found under conditions of erythroid hyperplasia in the bone marrow and especially when ineffective erythropoiesis occurs (megaloblastic anaemia, haemoglobinopathies, sideroblastic anaemia).

Staining of iron stores in the bone marrow with Perls’s reagent is a robust and relatively simple method for resolving difficulties that arise in the investigation of patients with suspected iron-deficiency anaemia. Although an examination of the amount of iron (usually graded semiquantitatively on a scale from 0 to 4, reflecting the strength of Prussian blue staining) does not provide any information as to the availability of the iron for haemoglobin formation, it does provide useful information as to the appropriateness of iron therapy for hypochromic anaemia. Bone marrow examination, moreover, may be diagnostic in patients suffering from hypochromic anaemias due to primary or sideroblastic change in the marrow, since the characteristic ring sideroblasts with or without other myeloblastic changes will be apparent.

In summary, the laboratory evaluation of patients with suspected iron deficiency should include a full examination of haematological parameters including microscopy of the blood film. Quantification of serum iron, serum transferrin, and transferrin saturation (TIBC) may be valuable in establishing the cause of the hypochromic or microcytic anaemia. Serum ferritin measurements are often confirmatory in the absence of malignant, hepatitic, or other inflammatory diseases—as may fluorimetric red-cell zinc protoporphyrin assays. Determinations of serum transferrin receptor concentration may provide evidence of increased demands for iron by the marrow or indeed expansion of the erythron, but this test is not universally available. Microscopic examination of a bone marrow aspirate, including staining with Perls’s reagent, may provide valuable information about iron stores in macrophages and the need for iron supplementation.

Disturbances of iron metabolism

Disorders of iron metabolism are common contributory factors in disease: iron deficiency is rife, particularly among those living in poverty, others whose access to meat is limited, and those in whom hookworm infestation occurs. At the same time, the prevalence of haemoglobinopathies and other anaemias such as myelodysplasia and sideroblastic syndromes that require transfusion and cause hyperabsorption of iron associated with ineffective erythropoiesis mean that iron storage disease also represents a major world health problem. In addition, hereditary haemochromatosis occurs at a high gene frequency in certain populations: this includes peoples of north European descent (adult haemochromatosis, due to mutations in HFE) and those of sub-Saharan African origin (African iron overload), in whom the nature of the predisposing gene is unknown.

Iron deficiency

About 30% of the world’s population—nearly 2 billion people—is anaemic, and at least half of these are believed to have iron-deficiency anaemia. Even in rich countries, such as the United States of America and Europe, up to 20% of menstruating women have signs of iron deficiency. In children and young adults, there is a frequency of between 5 and 10% of iron-deficiency anaemia—particularly in deprived socioeconomic groups.

Many population studies have in the past been based on erroneous attribution of anaemia solely to iron deficiency: there are many conditions, including the anaemia of chronic disorders and haemoglobinopathies such as β thalassaemia trait, that lead to hypochromic or microcytic red-cell indices. Population surveys based on the detection of iron-deficient erythropoiesis, especially those using determination of free red-cell zinc protoporphyrinconcentrations by fluorimetry, may enhance the detection of true iron-deficiency anaemia; determinations of serum ferritin concentrations also facilitate discrimination between the anaemia of chronic disease and true iron deficiency.

Causative factors

Iron-deficiency anaemia in populations is often attributed solely to an iron-poor diet, but in the absence of significant blood loss or intestinal parasites including hookworm, even the most iron-poor diets rarely cause iron-deficiency anaemia, except in growing children. The amount of iron required to repair obligatory losses is very small, so that at least 90% of the iron required for de novo haemoglobin formation in erythropoiesis is retrieved from senescent erythrocytes broken down by the mononuclear phagocyte system. Furthermore, once iron deficiency develops, striking adaptive changes occur in the absorptive mechanism for iron in the upper small intestine. In experimental animals with iron deficiency, mucosal expression of the divalent metal transporter 1 (DMT 1), on the brush border membrane of the intestinal epithelium, is induced. Iron-deficiency anaemia is also associated with enhanced intestinal expression of mucosal ferrireductase activity.

These changes do not represent the complete portfolio of adaptive changes that occur in response to iron deficiency. There is evidence that iron deficiency is associated with the recruitment of a greater length of mucosal surface in the upper small intestine for participation in the absorption of luminal iron. Iron deficiency, and the response to the removal of a unit of blood, may increase the overall absorptive efficiency of the intestine for iron up to tenfold—thus greatly enhancing the bioavailability of dietary iron.

Alcoholic beverages may provide a source of iron, and the absorption of haem iron present in red meat, poultry, and fish is usually between 15 and 35%. Between 2 and 20% of nonhaem iron present in fruit and vegetable sources is absorbed. Natural enhancers of iron absorption such as ascorbic acid, which maintains ferrous iron in its reduced form in the intestinal lumen, promote direct uptake by DMT 1. Fructose and other organic compounds of low molecular weight also form soluble and reduced complexes with iron released from nonhaem sources in food. In the West, normal individuals ingest between about 10 and 15 mg of iron daily. Adult men with normal iron stores absorb approximately 2% of the nonhaem iron ingested, whereas men with iron deficiency absorb more than 20% of iron from this source in the diet; the comparable figures for haem iron are 26 and 47%, respectively.

Many compounds present in the diet also inhibit or impede the absorption of iron released by digestion in the lumen. These compounds include tannin, especially present in tea, phytates present in bran and nuts, dietary fibre, and other inhibitory factors such as drugs, including tetracycline and alkalis. Some vegetarians of Asian origin ingest large amounts of phosphate and phytates which inhibit the absorption of iron provided in diets that may contain up to 30 mg of assayable total iron each day. A typical example is spinach which, although rich in iron, leads to the appearance of black stools when consumed in small or moderate amounts; these stools are black because of the passage of iron through the small intestine and its delivery to the colon where it forms insoluble ferrous sulphide complexes through the action of colonic sulphur-reducing bacteria.

Malabsorption of iron

The inability to release and absorb adequate amounts of iron from the diet is an important but unusual cause of iron deficiency. Disease of the stomach, duodenum, and upper jejunum may be responsible for the malabsorption of food iron, which may not be readily detected by studies involving the use of simple radioactive tracer measurements. On the other hand, properly conducted radioactive food labelling studies show that after gastric bypass surgery and after intestinal resection, there is malabsorption of nonhaem and haem food iron sources. Rarely, iron deficiency may result from inflammatory disease of the upper intestine that causes malabsorption: coeliac disease in infants and adults may be responsible, and the iron deficiency may be combined with deficiency of folic acid. Sometimes large pharmacological doses of iron with or without folic acid may overcome the anaemia caused by coeliac disease, but unless a strict gluten-free diet is instituted the anaemia recurs rapidly after iron therapy is stopped. Although malabsorption of food iron is an important aspect of the iron deficiency associated with coeliac disease, loss of iron exacerbates the effects of malabsorption. In coeliac disease this results from increased exfoliation of the epithelium in association with crypt hyperplasia and bleeding due to ulceration. The abnormal motility and maldigestion associated with upper gastrointestinal surgery compounded by anacidity caused by gastritis, or acid-suppressing agents, also impair the absorption of food iron. Bariatric surgery, which leads to diminished gastric acid secretion and bypasses the duodenum, is frequently complicated by iron-deficiency anaemia; prophylactic supplementation is thus recommended after this procedure, particularly in menstruating women.

Loss of iron

Women in the reproductive age group lose iron regularly at menstruation. An increased recommended daily allowance for women is higher than in all other groups: the average requirement for healthy menstruating women is approximately 1.4 mg of iron daily to replace losses, compared with normal men who lose about 0.8 to 0.9 mg of iron per day. Pregnancy is often associated with iron deficiency when growth of the fetus is rapid. Twin pregnancies and frequent childbirth, especially in women of low socioeconomic groups, are associated with iron-deficiency anaemia. Although anaemia is important, a very large study has been conducted that shows no reliable association between maternal anaemia and the complications of pregnancy, including preterm labour. Pregnancy itself is associated with the development of adaptive responses in the intestine and iron transport proteins that enhance the avidity of the gastrointestinal tract for bioavailable food iron. Clearly socioeconomic and sociopolitical considerations are likely to influence the population occurrence of iron deficiency in women of the reproductive age group, particularly since the investment of about 1 to 1.5 g of iron occurs with each pregnancy carried to term. This estimate includes blood loss associated with the birth and the investment of iron placed in human milk, which contains up to 0.5 mg/litre of iron bound to a whey protein, lactoferrin.

Other sources of iron loss

Intestinal parasites

Several hundred million people are heavily infested with hookworms. The two common hookworms of humans are Anclyostoma duodenale and Necator americanus. These helminths attach themselves to the lining of the small intestine by their buccal capsules and cause chronic blood loss by sucking blood from the intestinal villi. Hookworm infestation may be light, so that iron loss is not sufficient to cause iron deficiency. In hookworm disease, involving Old World and New World hookworms, heavy infestation occurs as a result of repeated exposure of the skin to contaminated soil. Mucosal immunity may also be reduced in the susceptible host. Although it is not known exactly what hookworms abstract from human blood, microscopic preparations show red cells expelled from the worm: each Anclyostoma induces the loss of up to 300 µl of blood daily, whereas each Necator causes the loss of up to 50 µl of blood. Clearly the occurrence of anaemia is dependent on the iron content of the diet, the extent of tissue iron stores, and the duration and intensity of the mucosal helminth infestation itself.

Since up to two-thirds of the haemoglobin iron released by the worms can be reabsorbed in the intestine, significant anaemia requires a very heavy parasite load; nonetheless, extremely severe anaemia may develop in patients with hookworm disease with all the attendant symptoms of fatigue, dyspnoea, palpitations, and mental changes—including pica. Nonspecific abdominal pain may occur and radiographic examination of the intestine or endoscopy may reveal duodenitis with a punctate inflammation associated with partial villus atrophy of the duodenojejunal mucosa. Oedema may result from cardiac failure in severe cases and also in association with hypoalbuminaemia, since heavy infestation may lead to significant protein-losing enteropathy. Hookworm disease may be associated with other helminth infections such as strongyloidiasis and ascariasis and itself may contribute to poor socioeconomic circumstances as a result of incapacity for work due to illness.

Hookworm parasites are widely distributed in southern Europe, Africa, the Middle East the Indian subcontinent, eastern Asia, and the New World, including southern United States of America. The heaviest infections usually affect rural workers in agricultural communities where repeated exposure occurs in isolated locations and where crops are harvested under conditions of poor sanitation. The iron-deficiency anaemia of hookworm disease may present difficulties for diagnosis when the mucosal inflammation that accompanies heavy infestation is associated with reduction in serum proteins such as albumin and transferrin; this, combined with an acute-phase response, may at first lead to a mistaken diagnosis of the anaemia of chronic disorders. Hookworm infestation as a cause of maternal anaemia is under-recognized, and this has inhibited the use of anthelmintic treatment in health provision for pregnant women. A recent study in sub-Saharan Africa estimated that nearly 40 million women of reproductive age are infected with hookworm; of these, about 7 million were pregnant in 2005. As expected, increasing intensity of infestation was associated with lower haemoglobin concentrations in pregnant women in poor countries. Given the number of pregnant women with hookworm and at risk of preventable hookworm-related anaemia, more studies on the potential benefit of anthelmintic treatment are warranted.

Other sources of blood loss

The gastrointestinal tract represents an important source of blood loss which should always be considered in patients with iron-deficiency anaemia. Ulcerating lesions of the small and large intestine, including cancers, are frequent causes of iron-deficiency anaemia. However, chronic intermittent bleeding can arise from unusual sources such as Meckel’s diverticula, angiodysplastic lesions, hamartomas, and other benign ulcerating tumours such as leiomyomas. Gastric ulcers may be associated with chronic intermittent bleeding, but duodenal ulcers rarely cause chronic gastrointestinal blood loss.

Oesophageal ulceration and inflammatory lesions can cause iron-deficiency anaemia, but precaution is needed in attributing blood loss sufficient to cause iron deficiency to such a source unless other potential sites of bleeding have been excluded. Other unusual sources of gastrointestinal bleeding include multiple telangiectatic lesions of Osler–Rendu–Weber disease (hereditary haemorrhagic telangiectasia)—in which bleeding may occur anywhere from the nasal or oropharynx down to the stomach and upper intestine. The blue bleb naevus syndrome, the Peutz–Jeghers syndrome, and other hereditary gut polyposes are rare causes of chronic gastrointestinal bleeding. Inflammatory disease of the lower small intestine and colon such as Crohn’s disease and ulcerative colitis, usually associated with chronic intestinal blood loss, may present with an abdominal history in which iron-deficiency anaemia is prominent. Very occasionally, artefactual iron-deficiency anaemia due to self-bleeding may occur; blood may be removed from any source but bizarre methods may be adopted to conceal it, thus requiring considerable ingenuity, and often detective work, to identify the cause. Because of the striking appearance of expectorated blood, iron-deficiency anaemia associated with frank haemoptosis requires little diagnostic skill, but occasionally recurrent intra-alveolar lung haemorrhage causes unexplained illness and anaemia. Occasionally, iron may be lost in the urine through the kidney in conditions where chronic intravascular haemolysis occurs. Losses may be sufficient to induce iron deficiency in the absence of marked changes in urine colour. Patients with haemolysis due to prosthetic or paraprosthetic cardiac valve malfunction may be revealed by the presence of characteristic red-cell changes; likewise in paroxysmal nocturnal haemoglobinuria, chronic intravascular haemolysis causes chronic urinary iron loss with or without visible haemoglobinuria. In these circumstances, free haemoglobin is released which quickly saturates the capacity of the plasma protein haemopexin to bind it; free haemoglobin spills into the glomerular filtrate where it is taken up by the proximal tubular cells and degraded. After degradation to haemosiderin, iron is lost in the urine when the iron-loaded epithelial cells are exfoliated.

Clinical and laboratory features of iron deficiency

Symptoms of iron deficiency include fatigue, pallor, palpitations, irritability, and little-recognized mental changes, such as pica. The patient may complain of a sore tongue, deleterious changes in the appearance of hair or hair loss, and angular cheilosis. Examination of the nails may reveal longitudinal ridging and, most often in elderly women with chronic iron deficiency for many years, koilionychia. There may be a complaint of dysphagia associated with the development of an oesophageal web (Patterson–Brown–Kelly or Plummer–Vinson syndrome). This again usually occurs in elderly or middle-aged women with chronic iron deficiency. A small proportion of patients with iron-deficiency anaemia have detectable but modest splenomegaly.

Diagnosis

Blood parameters will reveal microcytic anaemia usually in association with an unequivocal reduction in serum transferrin saturation (<16%) and a reduced serum ferritin concentration (<12 µg/litre). The absence of these features and of an acute-phase reactive response may suggest dyserythropoietic or sideroblastic anaemia or β thalassaemia trait. Lead poisoning may be associated with iron-deficient indices, with or without full-blown sideroblastic changes. A bone marrow aspirate stained with Perls’s reagent for iron in marrow macrophages will rapidly confirm reduced or absent stainable iron in the storage compartment and may also be revealing about other aspects of the anaemia, such as the presence of ring sideroblasts, dyserythropoietic features, and/or megaloblastic change.

The presence of immunoreactive serum transferrin receptors may provide additional evidence in favour of iron-deficiency anaemia, but because increased serum concentration of these receptors may be observed in several disorders of the bone marrow and the ELISA tests are relatively expensive, the role of this determination in the routine diagnosis of iron deficiency is not as yet established. Red-cell zinc protoporphyrin concentrations greater than 35 µg/dl of whole blood are usually observed in patients with iron deficiency; values greater than 100 µg/dl are generally associated with lead toxicity. Extremely high values may indicate the presence of erythropoietic protoporphyria or lead poisoning in the latter, free, rather than zinc protoporphyrin IX accumulates. Modest elevations in erythrocyte protoporphyrin can be observed in patients with haemolytic anaemias, sideroblastic anaemia, and occasionally, the anaemia of chronic disorders.

Investigations and management

The identification of iron-deficiency anaemia should be regarded in a sense as a symptom rather than a diagnosis of a patient’s malady: the management of those affected should always include an attempt to determine the cause. Common errors occur when, in elderly patients, iron deficiency is cynically ascribed to the presence of mild oesophagitis or gastritis observed at endoscopy, when the underlying cause is bleeding due to a coincidental but sinister gastrointestinal cancer elsewhere—and for which a diligent search is often required.

A full evaluation of the patient with iron deficiency should include an adequate dietary history including the consumption of drugs, such as aspirin and nonsteroidal anti-inflammatory drugs, which may be responsible for gastrointestinal bleeding. An enquiry should be made about additional gastrointestinal symptoms and other signs of blood loss; reasonable attempts should be made to evaluate the extent of menstrual loss, if the bleeding is to be ascribed to menorrhagia in women of reproductive age. Attention should be paid to the family history and a travel history to exclude causes such as hereditary haemorrhagic telangiectasia or hookworm disease.

Clinical examination should extend from an enquiry about previous gastrointestinal disease or surgery to an examination for visceral enlargement, abdominal lymphadenopathy, splenomegaly, and other features suggestive of intra-abdominal pathology such as portal hypertension and abdominal cancer. Hereditary haemorrhagic telangiectasia may be detected by the presence of the most subtle oronasal or palatal lesions.

With patients in whom the cause of the iron deficiency is not apparent, further studies may be needed to search for gastrointestinal bleeding, including detection of occult faecal blood on several samples taken consecutively. Endoscopic and radiographic studies of the gastrointestinal tract and serological studies for the presence of coeliac disease may be required and occasionally there is a need to quantify the amount of blood loss daily in the faeces or during menstrual flow by using radiolabelled chromium red-cell studies. In difficult cases, percutaneous visceral angiography of the coeliac and mesenteric arteries has proved invaluable for detecting sites of active gastrointestinal bleeding that are beyond the reach of conventional endoscopic procedures; in those patients who are actively bleeding, such a procedure can identify local sites of blood loss greater than 0.5 to 1.0 ml/min. The recent introduction of fibre optic double-balloon enteroscopy and wireless capsule endoscopy now offers powerful means to examine the entire small-intestinal mucosa extensively for the presence of bleeding lesions.

Meckel’s diverticulum is a potential cause of obscure gastrointestinal bleeding in young adults and children. Some Meckel’s diverticula can be diagnosed by scintigraphic studies using technetium-99m labelled pertechnetate which may be concentrated in the ectopic gastric mucosa. Meckel’s diverticulum and intestinal strictures, particularly in the ileum, may be occasionally revealed by retrograde colonic contrast radiographic studies. Other diagnostic tests include searching for endomysial (transglutaminase) antibodies, with confirmatory duodenojejunal biopsy to detect coeliac disease. Examination of the urine and sometimes sputum may be required to detect occult iron loss in exfoliated macrophages or proximal tubular cells, respectively, where intrapulmonary haemorrhage or renal iron loss is suspected.

Sometimes extensive diagnostic procedures fail to identify the cause of iron deficiency when occult gastrointestinal bleeding is responsible. Under these circumstances, it remains appropriate to conduct a diagnostic laparotomy, after consultation with an experienced surgeon, to identify the bleeding lesion. In adults of any age, an appreciable number of obscure gastrointestinal malignancies or treatable benign tumours can be identified by such a procedure which, when combined with angiography with or without enteroscopy, may permit identification of angiodysplastic lesions at remote sites. In younger adults and children, diagnostic laparotomy may be indicated to identify Meckel’s diverticula, intestinal stricture, and congenital abnormalities such as duplications that serve as occult sources of blood loss.

It is not unusual for the patient with recurrent chronic iron-deficiency anaemia to present a challenge for diagnosis. Even the most experienced physician would be well advised to consult widely with colleagues with expertise in radiology, nuclear medicine, and surgery before either prematurely abandoning the search for the causal lesion or for an ill-considered laparotomy without a thorough appreciation of the further difficulties it may pose.

Replenishing iron stores is but one aspect of the treatment of iron-deficiency anaemia. Iron should be replaced not only to restore the normal haemoglobin concentration but to replenish body iron stores. It is necessary to replace iron depleted in systemic tissues such as the muscles, where it is an essential component of cytochromes and other enzymes critical for optimal aerobic metabolism. Occasionally a therapeutic trial of oral iron for a defined period may be used to verify the suspected diagnosis of iron-deficiency anaemia. Adequate replacement of iron should be monitored for its effects: a reticulocyte response should be observed in peripheral blood maximally between the 7th and 10th days after initiating treatment and significant increases in blood haemoglobin concentration should be apparent within 2 to 4 weeks. If there is no evidence of continued blood loss, the haemoglobin concentration should come within the normal range within 2 months. Failure to meet these expectations suggests either that the anaemia is not caused by iron deficiency or that there is continued depression of bone marrow function—or that there is bleeding for which further investigation is needed.

Therapeutic preparations of iron

Iron salts should be administered by mouth unless there are overwhelming reasons for using the parenteral route—parenteral preparations of iron are associated with a greatly increased risk of toxicity. Iron–dextran complex and the newer iron–sucrose preparation are associated with hypersensitivity, including severe anaphylactoid reactions. Oral ferrous salts are better absorbed than ferric salts and show little difference amongst preparations in terms of rate of repair of anaemia at a given dosage of elemental iron.

It is usual to treat iron-deficiency anaemia with at least 100 to 200 mg of elemental iron daily. For full-blown iron-deficiency anaemia, ferrous sulphate is administered three times daily (equivalent to 3 × 65 mg of elemental iron). Some patients are unable to tolerate such a dose of iron because of constipation, diarrhoea, or abdominal pain; the presence of tarry, black stools may interfere with personal hygiene and thus lead to ultimate rejection of iron therapy by the patient. Under these circumstances the dose of iron may be reduced and this, rather than a change of iron salt preparation, usually improves tolerability. The frequency of unwanted effects with ferrous sulphate is the same as that of other iron salts when compared with the amount of elemental iron ingested. Once established, the optimal therapeutic response to oral iron increases the blood haemoglobin concentration by 0.1 to 0.2 g/dl per day. Replenishment of iron has a slow effect on the epithelial changes of iron deficiency and the atrophic glossitis may take several months to improve as iron stores are replenished.

Slow-release oral preparations of iron are available, which the manufacturers often claim release sufficient iron over a 24-h period for optimal haematological responses after once daily dosages. However, these preparations are likely to distribute the iron beyond the upper jejunum and thereby bypass those regions of the intestine in which iron absorption is most avid. Compound preparations of iron including B vitamins and folic acid are available, but there is little justification for prescribing these except for prophylactic use in pregnancy (see below). In infants and children, sugar-free preparations of iron complexes are available in the form of polysaccharide iron or iron–sodium EDTA (sodium ironedetate) complexes, which can be used as recommended by the manufacturer. In premature infants, up to 2.5 ml of syrup containing approximately 5 mg/ml may be used twice daily; up to 5 ml three times daily may be given to children aged 6 to 12 years.

Pregnancy

Prophylactic iron preparations are recommended in pregnant women who have risk factors for iron deficiency such as poor diet or prior menorrhagia, or those in whom gastric surgery has been carried out. Prophylactic iron may also be used in the management of infants of low birth weight including premature babies, twins, and infants delivered by caesarean section. Compound preparations of iron with folic acid may be used for the treatment of iron and folic acid deficiencies in pregnancy. For the prevention of neural tube defects in women planning a pregnancy, the United Kingdom Department of Health advises that a medicinal or food supplement of 400 µg/day of folic acid be taken before conception and during the first 12 weeks of pregnancy. Lone or combined iron compound preparations are not routinely indicated for prophylaxis in patients with chronic haemolysis or in renal dialysis since they may lead, in the circumstances of dyserythropoiesis, to chronic iron overload and secondary haemochromatosis.

Parenteral preparations of therapeutic iron

Given its potential toxicity, the only justification for the use of parenteral iron is in patients who are unable to cooperate with or tolerate oral iron therapy, or those with severe gastrointestinal disease that causes malabsorption or continuing severe blood loss. Provision of iron by the parenteral route does not normally lead to more rapid repair of anaemia than when adequate oral iron preparations are administered. Some patients with renal failure who receive haemodialysis have obligatory blood losses which cannot be treated adequately with oral iron preparations. These patients, and occasional patients receiving peritoneal dialysis, may require intravenous iron regularly. Two parenteral preparations of iron are now available in the United Kingdom: a ferric hydroxide–sucrose complex containing 20 mg/ml of iron (2%) and ferric hydroxide with dextran that consists of a colloidal stabilized preparation of iron containing 50 mg/ml. Severe sensitivity reactions to these agents may occur and facilities for cardiopulmonary resuscitation should be at hand with the use of either therapeutic iron preparation.

A very small test dose is recommended. Administration of these preparations should not be followed by oral iron therapy until at least 5 days after the last injection.

Unwanted and toxic effects of parenteral iron preparations

A history of allergic disorders including asthma, eczema, and prior anaphylaxis are regarded as contraindications to the use of parenteral iron, as is liver disease and concurrent infection. Moreover, these drugs are not recommended for children. Side effects include nausea, vomiting, taste disturbances, hypotension, parasthesias, abdominal disorders, fever, flushing, anaphylactoid reactions, and the reactivation of inflammatory arthropathy. Injection site reactions, including phlebitis, have been reported. Parenteral iron should probably be avoided in patients with pre-existing cardiac disease including arrythmias or angina. Parenteral iron is contraindicated in children below the age of 14 years.

Administration

Iron–dextran may be given by deep intramuscular injection into the gluteal region, as well as by slow intravenous injection. Iron–sucrose complex may be given slowly intravenously or by intravenous infusion. In both instances the total dose is calculated according to body weight and the presumed iron deficit set out in the manufacturer’s product literature. At least 15 min, with close observation, should elapse after the test dose before the therapeutic dose is administered.

General aspects of iron therapy

Treatment of causes of anaemia, including bleeding, is clearly a critical aspect of the management of iron-deficiency anaemia and its diagnosis. Coeliac disease should be treated with a gluten-free diet; bleeding lesions in the gastrointestinal tract may require definitive surgery directed to their healing. Occasionally, patients with a chronic bleeding disorder for which surgery is not indicated, such as hereditary haemorrhagic telangiectasia, may require long-term iron supplementation at doses less than that required to treat the acute iron-deficiency state. Periodic monitoring is required to ensure that the level of iron replacement is adequate to meet the demands of the bone marrow for de novo haem synthesis and that iron overload is not occurring. It should be recognized that relief of iron deficiency will improve many symptoms suffered by a patient even though they may suffer from an incurable underlying disease.

Treatment with iron should be continued until iron stores are replenished: there is no excuse for inadequate therapy, especially in those patients who are likely to suffer recurrent bleeding. Particular attention is needed for iron-deficient patients who have had episodes of acute bleeding treated by blood transfusion and who at the time of therapy are not anaemic. These patients require appropriate iron replacement to replenish iron stores for their long-term restitution of health. Because iron therapy leads to a reduction in the avidity of the transport system of the intestine for iron, it should be continued for several months after the anaemia has been corrected to re-establish appropriate iron stores, ideally as reflected by a serum ferritin determination within the normal range.

Unusual syndromes with iron-deficient erythropoiesis

Congenital deficiency of serum transferrin

There are a few reports of deficiency or virtual absence of serum transferrin in infants with disturbed growth, marked hypochromic anaemia, and disordered iron metabolism associated with systemic iron storage leading to tissue injury. This disease is extremely rare but holds great fascination for those investigators with an interest in the pathophysiology of iron metabolism. Profound deficiency of serum transferrin disturbs the normal ligand–receptor signalling mechanisms indicated in the overall control of body iron balance and absorption in the intestine. Hypo- or atransferrinaemia in humans appears to be inherited as an autosomal recessive trait; the gene encoding human serum transferrin maps to chromosome 3.

Studies of a naturally occurring mutant mouse, the hpx mouse, that also has deficiency of serum transferrin associated with runting and hypochromic anaemia due to iron-deficient erythropoiesis, indicate that the disorder responds to infusions of serum transferrin or plasma. These infusions restore normal growth and improve the abnormalities of iron homeostasis; iron-deficient erythropoiesis is also corrected, with resolution of the anaemia. The half-life of transferrin in the plasma is 5 to 10 days and so infusions of plasma or purified preparations enriched with transferrin can be administered at intervals. Since most individuals with transferrin deficiency produce limited amounts of the protein antigen, immune reactions to exogenous human transferrin appear to be either mild or rare. Absolute deficiency of transferrin receptors, e.g. as occurs in mouse embryos generated as a result of gene disruption technology in embryonic stem cells, is incompatible with normal development beyond the late embryo stage.

Other causes of refractory iron-deficient erythropoiesis

There are sporadic reports of iron deficiency occurring in children and adults for which no cause can be established after intensive investigation. In some instances the expected parameters of iron deficiency associated with iron-deficient erythropoiesis can be demonstrated in individuals who fail to respond to generous oral supplementation with iron salts; administration of parenteral iron, however, leads to an improvement in reticulocytosis with resolution of iron-deficient red-cell indices. Although at the time of writing no molecular lesions have been identified in any of the implicated iron and transport proteins, it is not impossible that disturbed function of DMT 1, ferroportin, hephaestin, or as yet uncharacterized moieties involved in the transport of iron across the intestine will be found in these disorders.

Occurrence of iron-deficient erythropoiesis in both females and males that responds only to parenteral iron supplementation is unlikely to be caused by hephaestin mutations, since this gene maps to the long arm of the X chromosome in humans. It is possible that acquired defects of the intestinal mucosa other than inflammatory disorders may contribute to malabsorption of therapeutic iron. Several young children have been reported with iron-deficiency anaemia refractory to oral therapy but which was corrected by parenteral supplementation. Careful investigation revealed an absorptive defect for iron which was corrected itself by systemic iron supplementation, and raises the possibility that severe iron deficiency itself prejudices the ability of the mucosal epithelium in the upper small intestine to carry out its normal absorptive function. However, no further investigations to identify the nature of this acquired metabolic defect have been provided. There is at least one well-documented instance of an acquired defect of iron delivery associated with signs of iron-deficient erythropoiesis caused by loss of human transferrin receptor function. This condition was associated with the development of antinuclear factor and other autoantibodies as part of an autoimmune illness in an adult woman with hypochromic anaemia. Autoantibodies directed against the transferrin receptor were identified in the serum of the patient, but the anaemia with its attendant sideropenia ultimately responded to a combination of steroids and azathioprine therapy; the titre of transferrin receptor autoantibodies of peripheral blood cells diminished. The extent to which this phenomenon occurs generally during the course of autoimmune disorders associated with anaemia is unknown.

Secondary iron storage disease (secondary haemochromatosis)

This is a worldwide problem. It occurs when excess iron is absorbed from the intestine or obtained by the breakdown of transfused red cells in the mononuclear phagocyte system. Each transfused unit of blood contains 200 to 250 mg of iron as haemoglobin. There are instances of iron storage disease occurring in patients who have received oral iron therapy over many years as medicinal tonics or as treatment for refractory anaemia. However, it is unknown if this would occur in the absence of another disorder, such as homozygosity for mutant alleles of the HFE gene that predisposes to iron storage disease, or underlying bone marrow disease. Conversely, iron excess may develop spontaneously in patients with haemolytic (and especially dyserythropoietic) anaemias alone, although it most commonly results from transfusion with or without underlying bone marrow disease.

Each millilitre of human blood contains the equivalent of 0.5 mg of elemental iron complexed with protoporphyrin. Iron present in transfused red cells is eventually retrieved after their breakdown in the macrophage system as a result of the actions of haem oxygenase, which releases bilirubin, carbon monoxide, and one atom of iron per haem molecule; thus each molecule of haemoglobin A yields four iron atoms. Although it is the mononuclear phagocyte system in which significant iron storage is first detected in transfused individuals, continued delivery of iron by this route leads to the excess of iron-loaded ferritin and its breakdown product, haemosiderin, in parenchymal cells throughout the body, with ensuing tissue injury and functional impairment. After the transfusion of 15 to 20 units of blood (representing about 5 g of elemental iron), iron toxicity occurs.

In dyserythropoietic anaemias such as thalassaemia and sideroblastic anaemia, symptoms and signs of iron storage disease may develop early in life and are related to increased dietary iron absorption by the intestine. Although some patients with β thalassaemia intermedia are treated by occasional transfusion, much of the excess iron stored in the body originates from ingested rather than transfused iron. Iron absorption in healthy adults amounts to 1 to 2 mg/day, but in β thalassaemia intermedia this may be increased more than fivefold. In regularly transfused patients with β thalassaemia major, the massive expansion of the erythropoietic marrow may be suppressed to render absorption of iron normal or near normal. However, in patients with thalassaemia who are transfused only intermittently, erythroid hyperplasia persists and excessive absorption of iron from the diet contributes significantly to the iron storage derived from transfused cells; several grams of additional iron may thus be acquired each year.

Patients with hypochromic anaemias due to sideroblastic change in the marrow are particularly at risk because they may be misdiagnosed as suffering from chronic or recurrent iron-deficiency anaemia; they thus receive long-term supplementation with oral iron that serves merely to exacerbate the iron-loading state. It is noteworthy, however, that patients with haemolytic anaemia due to sickle cell haemoglobin C disease do not commonly develop iron overload as a result of enhanced iron absorption: iron storage disease is thus generally restricted to transfused patients with chronic anaemias. Particular difficulties arise in refractory anaemias in which there is a hyperplastic bone marrow with ineffective erythropoiesis which appears to drive the inappropriate absorption of iron by the intestine.

In the South African Bantu people, the excess iron is ingested in an unusually bioavailable form in beers and other alcoholic drinks prepared by fermentation in iron pots (kaffir beers). Soluble complexes of readily bioavailable iron in these drinks contribute to secondary haemochromatosis, which is common in men in this population and other related sub-Saharan African populations. Although much of the iron is at first detected in the mononuclear phagocyte system (and is seen particularly in Kupffer’s cells on liver biopsy), associated hypogonadism and vitamin C deficiency later induce scurvy and osteoporosis. Dietary adjustment and iron chelation therapy may relieve the disorder, which is becoming less common after its recognition in the early 1950s. It is of interest that family studies point to a genetic component which predisposes individuals to this secondary iron storage disease within given pedigrees.

The nature of the stimulus leading from the excess iron turnover that accompanies hyperplastic bone marrow to the intestinal disturbance is unknown. The degree of excess iron absorption is however related to the extent of expansion of the red-cell precursor population: blood transfusions, which suppress the marrow, decrease the absorption of food iron. The toxic properties of iron appear to be related to its capacity to participate in free-radical-generating reactions that form reactive oxygen and nitrogen intermediates implicated in cellular and tissue injury.

Clinical features

The clinical features of secondary iron storage disease in children with chronic anaemias closely resemble hereditary forms of juvenile haemochromatosis. Iron accumulates rapidly in the liver and in the endocrine glands. The several hundred gonadotrophs present within the anterior pituitary gland appear to be particularly susceptible to iron toxicity and hypogonadotrophic hypogonadism results. Iron also accumulates in the β-cells of pancreatic islets, leading to diabetes; in the zona glomerulosa of the adrenal glands, with adrenal failure and mineralocorticoid deficiency; and in the parathyroid glands, ultimately causing hypoparathyroidism. Secondary iron storage disease also has a predilection for the myocardium. This causes sudden death as a result of tachyarrythmias and injury to cardiac conducting tissue or cardiomyopathy with intractable cardiac failure. Secondary iron storage disease in β thalassaemia and congenital dyserythropoietic anaemias is thus characterized by progressive myocardial disease, endocrine failure, and infantilism.

Untreated iron storage disease is the most common cause of death in these disorders. Similar manifestations of iron toxicity are observed in other patients with secondary iron storage in which the accumulation of iron is less rapid. A picture resembling full-blown adult haemochromatosis ultimately supervenes with diabetes and cirrhosis (sometimes complicated by transfusion-related viral hepatitis and the formation of hepatocellular carcinomas) in the presence of deep skin pigmentation. Secondary iron storage disease represents a significant threat to well-being and prognosis in the chronic anaemias. Once cardiac arrythmias have developed, the outlook is usually bleak and urgent chelation therapy with parenteral desferrioxamine is indicated.

Diagnosis

Secondary iron storage disease should be suspected when the saturation of serum transferrin is greater than 60%. In established secondary iron storage disease, there is a raised nontransferrin iron-binding fraction which may contribute to the tissue injury, since the amount of circulating iron may exceed the binding capacity of circulating transferrin. Under these circumstances, transferrin saturation is usually measured at greater than 90 to 95% and is accompanied by an elevation of serum ferritin which, in the absence of active liver disease, faithfully reflects the extent of iron storage disease and the risks of iron-mediated damage.

Iron chelation therapy should probably be introduced at serum ferritin concentrations greater than 1000 µg/litre or if there is biopsy evidence of excess iron storage or a transfusion load of more than 15 units of exogenous red blood cells. Diagnostic evidence of iron storage may be obtained from biopsies of the liver or myocardium; skin biopsy shows excess iron in the sweat gland acini and perifollicular apocrine glands together with increased melanin deposition. In biopsy samples of the liver and heart, histochemical iron storage can be quantified by chemical iron estimations: often the liver iron content exceeds 2% of tissue dry weight (normally less than 0.14% or approximately 7 mg of iron per gram dry weight). Iron concentrations may exceed 5% in affected tissues such as endocrine glands and the pancreas. Liver biopsy may facilitate staging of the disease, particularly in relation to coincidental viral hepatitis where the presence of fibrosis and cirrhosis combined with iron deposits in the parenchymal cell may contribute useful prognostic information. In patients in whom tissue biopsy determinations are not possible, an estimate of body iron overload may be gained by injection of a single dose of 500 mg of desferrioxamine intramuscularly and collection of urine for 24 h in an iron-free plastic container; the daily excretion of more than 2 mg of the coloured ferrioxamine–iron complex indicates iron excess.

Although serum ferritin concentrations generally reflect the amount of iron stored in the tissues, there is a poor correlation between the levels of ferritin in iron-overloaded subjects and clinical outcome. Ferritin concentrations in serum are subject to wide variations; as a result of infection or inflammation (when as an acute reaction it is spuriously elevated), and ferritin concentrations may be reduced when vitamin C is deficient. In contrast, since the liver is the principal site of the iron storage, hepatic iron concentrations provide useful guidance as to prognosis overall, including outcomes from iron-induced cardiac injury, fatal complications of which are usually observed in patients when tissue iron exceeds 1.5% of dry liver weight. In specialized centres, noninvasive methods have been developed to measure liver iron concentrations, including whole-body magnetic susceptibility techniques but neither this nor sophisticated T 2-weighted MRI of the heart or liver has been so far generally accepted in practice. Conventional T 2-weighted imaging may provide a crude assurance that iron storage is either present or under control but is too insensitive to contribute to serial monitoring of secondary iron storage disease—except for the investigation of potential complications such as hepatocellular carcinoma.

Treatment

Patients with homozygous β thalassaemia, sickle cell disease, and related conditions such as myelodysplasia, who are transfusion dependent require adequate blood transfusion to maintain a normal or near normal haemoglobin concentration combined with an iron-chelating agent. Long-term studies provide compelling evidence over 30 years that survival in iron-loaded β thalassaemic subjects is greatly enhanced by treatment with subcutaneous desferrioxamine, which prevents and reverses the cardiac manifestations of iron storage disease. It must be noted, however, that full compliance with this demanding treatment is required for benefit to accrue, which requires equal commitment from the patient and attending medical and nursing personnel alike. Splenectomy or bone marrow transplantation may be considered in certain cases but is beyond the scope of this article. The overall outcome and prognosis for β thalassaemia has also been improved by screening donor blood for HIV and hepatitis B and C viruses, as well as other pathogens. These factors are ancillary but may potentiate the development and complications of secondary iron storage disease.

The primary goal of chelation therapy is to remove iron at the rate that it is accumulating, but the effectiveness of this approach is limited because, with the exception of the liver, only a fraction of the tissue iron pool is accessible to chelating agents. This applies particularly to iron in the key organs such as the heart, from which tissue iron extraction is slow. In general the most efficient regimens for removing iron depend on continuous provision of the chelating agent. This can only be achieved with difficulty in the case of desferrioxamine, since the drug has a short plasma half-life. A new orally active chelator, deferasirox, is able to achieve continuous removal of iron after a single daily dose. This agent is able to attain trough concentrations which decrease labile iron species in the plasma persistently over time. Another agent, deferiprone, is also orally active but neither it nor desferrioxamine appears to be as effective as deferasirox.

The preferred route for desferrioxamine mesilate administration is by slow subcutaneous infusion over 12 to 16 h for up to 7 days/week; this is usually done on an ambulatory basis in adults but nocturnal administration is used particularly in children. Nocturnal administration relies on the use of slow clockwork or battery-operated infusion devices. Although electrical syringe pumps are in common use (such as the Graseby driver device), smaller quieter infusion devices (such as the Cronoject) are now available. Light precharged balloon pumps manufactured by Baxter, though expensive, are also in use. The total daily dose of desferrioxamine is usually set at 20 to 30 mg/kg of body weight in children with the maximum usually determined by the extent to which near-saturated solutions of the drug can be tolerated by the patient; in established iron overload the daily dose is usually 20–50 mg/kg. Additional benefit has been suggested from the intravenous administration of up to 2 g desferrioxamine with each unit at the time of blood transfusion: it is important that the drug is not added to the blood but it may be coadministered through a separate intravenous line given through the same cannula. In patients without cardiac disease it has been shown that the daily oral administration of ascorbic acid at 2 to 3 mg/kg increases the amount of iron that can be chelated by desferrioxamine. Serial determinations of serum ferritin concentrations, combined with regular clinical monitoring and assessment of cardiac, hepatic, and endocrine function assist in the assessment of iron storage disease and the efficacy of iron chelation therapy. Periodic echocardiograms and electrocardiography, with 24-h ECG monitoring, are desirable aspects of management. Urinary excretion of the coloured ferrioxamine complex can be easily measured by light spectroscopy. Desferrioxamine promotes not only urinary excretion of iron but also chelates iron from the body stores, which is excreted into the faeces via the biliary system.

Several studies show that patients with β thalassaemia maintained on adequate transfusion regimes who are able to tolerate their infusions of subcutaneous desferrioxamine grow and develop normally and have a better prognosis than those who either default from or do not comply fully with the chelation regimen. When treatment is initiated, careful monitoring is needed using 24-h urine collections for iron measurements to judge the excretion of iron as the dose of desferrioxamine is escalated. Recently, the use of T 2*-weighted cardiac MRI studies to monitor iron overload in heart tissue has proved to be particularly valuable and allows for intensification of chelation regimens in high-risk patients. Daily doses of desferrioxamine may be increased to about 50 mg/kg of body weight; this usually represents the maximum that can be tolerated. In infants and growing children, unless severe cardiac disease or iron overload is present, the dose should not exceed 35 mg/kg per day over 5 nights each week. Thereafter, most well-transfused patients with β thalassaemia can be maintained in negative iron balance by the use of not more than 40 mg/kg. For patients who receive blood transfusions, a single intravenous infusion of desferrioxamine given separately from but at the same time as each blood transfusion, at a dose of approximately 150 mg/kg of body weight, also contributes to the control of iron storage disease. In patients who develop endocrine failure, prompt replacement of deficient hormones should be introduced. Sex-steroid hormone replacement may relieve infantilism and improve self-esteem in developmentally arrested adolescents and children.

Desferrioxamine is usually well tolerated and, apart from minor skin reactions, is remarkably nontoxic. These reactions can usually be controlled by lowering the concentration of the drug in the infusion and by alternating sites of infusion; hydrocortisone in doses of up to 10 mg has been reported to reduce severe cutaneous reactions. Very high doses of desferrioxamine, particularly those used for treatment of life-threatening cardiac iron overload and given by intravenous rather than subcutaneous infusion (see below), have been associated with retinal injury and lens opacities as well as hearing loss. Since high-tone hearing loss may occur also, it may be prudent to monitor visual acuity and auditory function at intervals during treatment over the years for which desferrioxamine is required. Minor gastroenterological disturbances, myalgia, and very rarely anaphylaxis may occur; rapid administration of desferrioxamine may be associated with hypotension, especially when given intravenously. Desferrioxamine interacts unfavourably with phenothiazines and coma may result from its use in patients receiving these agents. Some patients receiving desferrioxamine develop infections with microorganisms such as yersinia and fungi such as mucor that have fastidious requirements for iron. Iron-overloaded patients may also develop other systemic microbial infections and are particularly susceptible to infections with the marine vibrio, V. vulnificus. It seems likely that under these circumstances the desferrioxamine may serve, as nature intended, as a source of iron for uptake by microbial siderophore systems. In patients with acute or subacute cardiac manifestations of iron overload, there are encouraging reports of the effects of high-dose intravenous desferrioxamine: desferrioxamine may reverse cardiac failure and life-threatening tachyarrythmias.

For many patients the emerging orally active tridentate ferric iron chelator deferasirox offers an attractive option for once-daily therapy without the discomfort and limitations of continuous subcutaneous infusions. Deferasirox is now licensed in many countries including the United States of America and Europe. In Europe it is recommended for the treatment of chronic iron overload in adults and children over the age of 6 years with thalassaemia major who receive frequent blood transfusions (>7 ml packed red cells/kg per month). Deferasirox is also licensed for chronic iron overload where desferrioxamine is contraindicated or inadequate in thalassaemia major requiring less frequent transfusions, in patients with other anaemias, and in children aged 2–5 years. The dose is 20–30 mg/kg once daily according to the extent of iron overload (as judged by transfusion history and serum ferritin concentrations). Dosage adjustments should be made every 3–6 months. The drug is a very powerful chelator, and annual ear and eye examinations are required as well as height and sexual development in children. Monitoring of baseline and monthly hepatic and renal function (including tests for proteinuria) is required; at the start of therapy, weekly monitoring is recommended. Although deferasirox may cause proteinuria and headache as well as gastrointestinal effects, and less commonly visual and hearing disturbances occur as well as acute renal failure, these unwanted effects appear to be only rarely encountered. Deferasirox 20 or 30 mg/kg per day has been shown to have a beneficial effect on liver iron concentrations and serum ferritin concentrations, and there are several reports of acceptable tolerability with regular patient monitoring. At the time of writing, longer-term efficacy and safety data are required but as an oral iron chelator the agent appears to offer unique advantages for the management of secondary iron storage disease.

Deferiprone, a novel bidentate an oral iron chelator of a different chemical class from the naturally occurring bacterial agent desferrioxamine, has recently been licensed in Europe for treatment of iron overload in patients unable to tolerate desferrioxamine or in whom it is contraindicated. This drug, of the hydroxpyridone class, is used at a dose of 25 to 100 mg/kg of body weight daily in three divided doses; it is not recommended for children under the age of 6 years. Deferiprone appears to induce overall negative body iron balance in patients with severe homozygous β thalassaemia, with attendant reductions in serum ferritin concentrations, and clearly represents the first newly licensed oral drug with this important indication. In a proportion of patients, however, negative iron balance does not appear to be maintained and the drug may cause serious toxicity including neutropenia and the occasional incidence of agranulocytosis which appears to be mediated by an immune mechanism.

The use of deferiprone remians somewhat controversial following a report that its continued administration may be associated with progressive hepatic fibrosis. Conversely, despite the inconvenience of its use, long-term studies of patients receiving desferrioxamine for iron storage disease in homozygous β thalassaemia show that it is largely safe; moreover, desferrioxamine improves cardiac function and life expectancy and arrests hepatic fibrosis in secondary haemochromatosis. Recently, a randomized, placebo-controlled, double-blind trial of the effect of combined therapy with desferioxamine and deferiprone on myocardial iron in thalassemia major using cardiovascular MRI has been reported. This study shows that the combination of deferiprone and desferioxamine improved cardiac iron deposition more than monotherapy. However, direct comparisons between deferiprone and deferasirox have yet to be undertaken. Safety information and a side-effect profile on the use of deferiprone at a daily dose of 75 mg/kg is available; at the time of writing the drug is not approved by the United States Food and Drug Administration.

Other aspects of care

The single most important aspect of care is compliance with iron chelation therapy and monitoring, especially for infants and other young patients with iron-loading anaemias such as thalassaemia. Regular attendance of special clinics is advisable so that wide-ranging professional support from familiar personnel can be given to reinforce medical care delivered with attention to continuity and the nurturing of independence.

Patients with secondary iron overload should be monitored not only for the progression of their iron storage as determined by parameters of iron metabolism, but also clinically for the presence of iron-mediated tissue injury. Regular echocardiography, electrocardiography, hormone measurements, and physical examinations are required to search for the presence of endocrine failure, including hypoparathyroidism and adrenocortical failure, both of which may be very difficult to detect. Patients with evidence of hypogonadism should be treated with hormone supplementation to ensure normal sexual characteristics, and vigilance should be maintained for the development of diabetes mellitus. Psychological difficulties are prevalent in children and adolescents receiving iron-chelation therapy and transfusion for chronic anaemias and appropriate counselling is often needed over long periods to build up trust with them and their families and to maintain compliance with treatment. Patients with established infantilism and stunted growth frequently develop skeletal disease in addition to that related to their marrow disorder, and investigations should be carried out to search for osteopenia and osteoporosis for which additional therapy will be needed. Bone disease and growth arrest may be caused by the overenthusiastic use of desferrioxamine in young infants, and in these patients the daily dose of desferrioxamine should be reduced to below 40 mg/kg, which usually restores growth velocity to normal.

Finally, patients with secondary iron storage disease should be advised to moderate their dietary intake of iron-rich foods such as meat: some investigators advocate the drinking of strong tea at meal times, especially in patients with thalassaemia intermedia. This tannin-rich drink has been shown to decrease bioavailability of dietary iron and should improve overall iron balance in this at-risk group. As far as possible, the blood haemoglobin concentration should be maintained in the normal range to ensure growth and responsiveness to hormone supplements; patients with significant transfusion requirements should be considered for splenectomy when they reach an age of over 5 years. As with patients who are not iron overloaded, splenectomized individuals should be treated appropriately by immunization and antimicrobial prophylactic therapy as far as possible to reduce the risk of intercurrent bacterial infection. This risk is potentiated by systemic iron storage.

Treatment of severe cardiac manifestations of iron storage disease

Continuous intravenous infusions of desferrioxamine not exceeding 50 to 60 mg/kg daily are now recommended for life-threatening heart disease. High-dose intravenous infusions may cause unacceptable toxic injury, especially in the retina and inner ear. Desferrioxamine given continuously through a permanent indwelling portable catheter within the superior vena cava, with careful attention to sepsis, is a satisfactory method for securing reversal of cardiac disease in high-risk patients with serum ferritin concentrations that persist at greater than 2500 µg/litre or who have hepatic iron concentrations that exceed 1.5% of dry liver weight. Improved outcomes have been reported with the use of anticoagulation induced by warfarin, and scrupulous attention to cutaneous needle resiting and skin care to reduce the risk of thrombosis and complicating infections.

Pregnancy

Desferrioxamine therapy is not recommended by the manufacturer during pregnancy but, despite this, many successful pregnancies have been reported without fetal injury. The drug should probably be avoided during the middle trimester and should almost certainly be avoided, because of unknown teratogenicity, in early pregnancy or at the time of any planned conception. None the less, it may be reasonable to restart desferrioxamine therapy in the final trimester of pregnancy if the risks to the mother from iron storage disease are high. No information is available on deferasirox or deferiprone in pregnancy and these drugs should probably not be used until more experience is forthcoming.

Prognosis and outcome

The principal causes of death in secondary iron storage disease include cardiac failure and arrythmias, endocrine failure and the consequences of diabetes mellitus, infection, and hepatocellular carcinoma. Unless treated, secondary haemochromatosis is a rapidly fatal disease when associated with transfusion therapy and intestinal hyperabsorption of iron in the chronic anaemias. Less than one-third of those unable to comply with iron chelation therapy survive with β thalassaemia major to the age of 25 years. However, the outcome of iron storage disease in patients with chronic anaemia is now greatly improving, with enhanced life quality and duration. One study has indicated that 95% of patients with β thalassaemia who administer desferrioxamine subcutaneously more than 250 times each year will survive to 30 years; whereas only 12% of those who do not will survive to this age. In the United Kingdom the overall survival is 50% at 35 years, but at one specialist centre the actuarial survival in more than 100 patients was 80% at 40 ears. Continuous intravenous desferrioxamine can be claimed to reverse life-threatening arrythmias in cardiac iron overload and also improve or reverse left ventricular or biventrical heart failure in a majority of cases. One report describes the actuarial survival of more than 60% at 13 years of patients with life-threatening disease and β thalassaemia so treated; this outcome appears to be accompanied by improved cardiac tissue iron signals on MRI.

This again emphasizes the benefits of care administered at a dedicated treatment centre. From 1980 to 1999 there were 12.7 deaths from all causes per 1000 patient years. In 2000–2003, the death rate from all causes fell significantly to 4.3 per 1000 patient years. This was mainly accounted for by the reduced death rate from iron overload, which fell from 7.9 to 2.3 deaths per 1000 patient years. Several reports also show that the frequency of hypogonadism, diabetes, and growth retardation is significantly reduced by effective iron chelation.