Residency · Residency · Hematology Thrombosis

Iron Deficiency Anemia - Beyond the Basics

Introduction

Iron deficiency is the most common nutritional deficiency worldwide, affecting approximately two billion people across all socioeconomic strata. Iron deficiency anemia (IDA) represents the severe end of a clinical spectrum that includes iron depletion (reduced stores without functional consequences), iron-restricted erythropoiesis (insufficient iron supply to the marrow despite some residual stores), and finally frank anemia with microcytic hypochromic red cell indices. The fellowship-level clinician must master not only the straightforward diagnosis and treatment of classic IDA but also the nuanced diagnostic scenarios encountered in inflammatory states, chronic kidney disease, and heart failure, as well as the rapidly evolving landscape of intravenous iron therapeutics.

Iron Metabolism - Advanced Concepts

Iron Absorption and Transport

Dietary iron exists in two forms with markedly different absorption characteristics. Heme iron, derived from hemoglobin and myoglobin in animal products, is absorbed with an efficiency of 15 to 35 percent via the heme carrier protein 1/proton-coupled folate transporter (HCP1/PCFT) on the apical surface of duodenal enterocytes. Non-heme iron, found in plant-based foods and fortified products, is absorbed with a much lower efficiency of 2 to 20 percent, and its uptake requires reduction from the ferric (Fe3+) to the ferrous (Fe2+) state by duodenal cytochrome B (DcytB) before import through divalent metal transporter 1 (DMT1).

Once inside the enterocyte, iron may be stored as ferritin or exported across the basolateral membrane into the circulation by ferroportin (SLC40A1), which is the only known cellular iron exporter in the body. This singular dependence on ferroportin for iron export from all cell types makes it the critical control point for systemic iron homeostasis. After export, ferrous iron must be oxidized back to the ferric state for loading onto transferrin, the primary plasma iron transport protein. This oxidation is performed by hephaestin at the enterocyte basolateral surface and by ceruloplasmin in the systemic circulation. Each transferrin molecule can bind two Fe3+ ions, and under normal physiologic conditions, transferrin saturation (TSAT) ranges from 20 to 45 percent.

Hepcidin - The Master Regulator

Hepcidin, a 25-amino acid peptide hormone produced by hepatocytes, is the central regulator of systemic iron homeostasis. Its mechanism of action is elegantly simple: hepcidin binds directly to ferroportin on the surface of enterocytes, macrophages, and hepatocytes, causing ferroportin to be internalized and degraded. This effectively blocks iron export from these cells, trapping iron intracellularly and reducing plasma iron availability.

Hepcidin production is increased by inflammatory cytokines, particularly interleukin-6 acting through the JAK-STAT3 signaling pathway to activate transcription of the HAMP gene encoding hepcidin. Iron overload also stimulates hepcidin through the BMP6/SMAD pathway, as does infection. Conversely, hepcidin is suppressed by iron deficiency, hypoxia acting through the HIF pathway, increased erythropoietic drive, and testosterone.

A particularly important negative regulator of hepcidin is erythroferrone (ERFE), a hormone produced by erythroblasts in response to erythropoietin stimulation. Erythroferrone suppresses hepcidin by sequestering BMP ligands, thereby ensuring adequate iron supply to the marrow during periods of heightened erythropoietic activity. This mechanism explains why patients with thalassemia and other conditions of ineffective erythropoiesis develop paradoxical iron overload despite not receiving transfusions: massively expanded but ineffective erythropoiesis drives excessive erythroferrone production, chronically suppressing hepcidin and increasing intestinal iron absorption.

Cellular Iron Handling

At the cellular level, iron uptake and utilization are tightly regulated. Transferrin receptor 1 (TfR1) is ubiquitously expressed and mediates iron uptake by endocytosis of the transferrin-TfR1 complex. Its expression is inversely related to cellular iron status, and the cleaved form of TfR1, soluble transferrin receptor (sTfR), circulates in the plasma and serves as a clinically useful marker of iron demand, particularly by the erythroid compartment. Transferrin receptor 2 (TfR2) is predominantly expressed on hepatocytes and functions as an iron sensor that modulates hepcidin transcription rather than mediating iron uptake.

Ferritin serves as the primary intracellular iron storage protein, capable of sequestering up to 4,500 iron atoms per molecule in a non-toxic, bioavailable form. Serum ferritin, which is largely released by macrophages, reflects total body iron stores under most conditions. The IRE/IRP system provides an elegant post-transcriptional regulatory mechanism: iron regulatory proteins (IRP1 and IRP2) bind to iron-responsive elements (IREs) in the untranslated regions of mRNAs encoding ferritin, TfR1, ferroportin, and DMT1, coordinately adjusting the expression of these proteins in response to intracellular iron levels.

<image>A detailed diagram of systemic iron homeostasis showing the central role of hepcidin. Illustrate the duodenal enterocyte with DMT1 on the apical surface and ferroportin on the basolateral surface. Show ferroportin also on macrophages (recycling iron from senescent RBCs) and hepatocytes. Depict hepcidin being released from the liver and binding to ferroportin, causing its internalization. Include regulatory inputs to hepcidin: IL-6/JAK-STAT3 pathway (inflammation), BMP6/SMAD pathway (iron stores), HIF pathway (hypoxia), and erythroferrone from erythroblasts (erythropoietic drive). Show transferrin in the circulation carrying Fe3+ to the bone marrow. Use arrows to indicate stimulatory and inhibitory signals. Color-code the pathways: red for stimulatory, blue for inhibitory. Medical textbook illustration style.</image>

Diagnosis of Iron Deficiency

Traditional Iron Studies

The traditional iron panel includes serum ferritin, transferrin saturation (TSAT), total iron-binding capacity (TIBC), and serum iron. In straightforward iron deficiency, ferritin is low (below 30 ng/mL), TSAT is reduced (below 20%), TIBC is elevated (reflecting compensatory upregulation of transferrin synthesis), and serum iron is low. In the anemia of chronic disease (ACD), ferritin is typically elevated above 100 ng/mL because iron is trapped in macrophages by hepcidin-mediated ferroportin degradation, but TSAT remains low as iron delivery to the marrow is impaired. The most diagnostically challenging scenario is the coexistence of iron deficiency and chronic disease, in which ferritin falls into an intermediate range (30 to 100 ng/mL) and additional markers are needed for accurate diagnosis. In iron overload, ferritin exceeds 300 ng/mL, TSAT rises above 45%, and TIBC is low.

Ferritin Thresholds - Context Matters

The interpretation of serum ferritin must be adjusted for clinical context because ferritin is an acute phase reactant whose concentration may be significantly elevated by inflammation, liver disease, malignancy, and infection. In the outpatient setting in an otherwise healthy individual, a ferritin below 30 ng/mL is diagnostic of iron deficiency with a sensitivity of 92% and specificity of 98%. In patients with inflammatory states or chronic kidney disease, a ferritin below 100 ng/mL strongly suggests coexistent iron deficiency. The European Society of Cardiology defines iron deficiency in the context of heart failure as a ferritin below 100 ng/mL, or a ferritin of 100 to 299 ng/mL combined with a TSAT below 20%. Importantly, ferritin can exceed 500 ng/mL in the setting of liver disease, malignancy, or infection despite true underlying iron deficiency, making it essential to use additional diagnostic tools in these patients.

Advanced Diagnostic Tools

When traditional iron studies are ambiguous, several advanced markers can provide diagnostic clarity. The soluble transferrin receptor (sTfR) is elevated in iron deficiency because it reflects increased erythroid iron demand and TfR1 expression. Critically, sTfR is not affected by inflammation, making it particularly valuable in distinguishing iron deficiency from the anemia of chronic disease. The sTfR/log ferritin index (also known as the Thomas plot) combines the strengths of both markers: a ratio greater than 2 suggests iron deficiency, and a ratio above 3.2 is highly specific. This index is the most useful single calculation for diagnosing coexistent iron deficiency in the setting of inflammation.

Reticulocyte hemoglobin content (CHr or Ret-He), measured on automated hematology analyzers, reflects the hemoglobin content of the most recently produced reticulocytes and provides a real-time assessment of iron availability over the preceding 24 to 48 hours. A value below 28 pg indicates iron-restricted erythropoiesis. The percentage of hypochromic red cells (%HYPO), when greater than 5%, suggests functional iron deficiency, and values above 10% are strongly supportive. Hepcidin levels, which are low in iron deficiency and elevated in the anemia of chronic disease, represent an emerging diagnostic tool but are not yet widely available in clinical practice. The bone marrow iron stain with Prussian blue, demonstrating absent stainable iron in macrophages, remains the definitive gold standard for diagnosing iron deficiency, but it is rarely needed when non-invasive tests are used properly.

Red Cell Indices in IDA

The classic microcytic (MCV below 80 fL) hypochromic anemia of iron deficiency is a late finding, occurring only after iron stores have been substantially depleted and iron-restricted erythropoiesis has been ongoing for sufficient time to alter the red cell population. The red cell distribution width (RDW) is often elevated early in the course of iron deficiency (above 14.5%) and may be the first CBC abnormality to appear, reflecting the heterogeneity of the red cell population as microcytic cells are produced alongside previously normal cells. The Mentzer index, calculated as MCV divided by RBC count, provides a quick heuristic for distinguishing iron deficiency anemia (index greater than 13) from thalassemia trait (index less than 13), though its sensitivity is only approximately 80%. Reactive thrombocytosis is commonly observed in IDA, resulting from homology between thrombopoietin and erythropoietin and from the stimulatory effect of low iron levels on megakaryopoiesis.

Etiology - Systematic Approach

Blood Loss (Most Common in Adults)

Blood loss is the most common cause of iron deficiency in adults and demands a thorough etiologic investigation. Gastrointestinal blood loss is the leading cause in males and postmenopausal females, and occult GI malignancy must always be excluded in these populations. Other GI sources include peptic ulcer disease, angiodysplasia, Cameron erosions in large hiatal hernias, celiac disease, and inflammatory bowel disease. Genitourinary blood loss, particularly menorrhagia (defined as menstrual blood loss exceeding 80 mL per cycle), is the most common cause in premenopausal women, and von Willebrand disease should be screened for in cases of heavy menstrual bleeding. Iatrogenic causes include diagnostic phlebotomy, hemodialysis (which results in a loss of 1 to 3 grams of iron per year), and surgical blood loss. Pulmonary causes, including diffuse alveolar hemorrhage and pulmonary hemosiderosis, are less common but should be considered when other sources are not identified.

Malabsorption

Iron malabsorption can result from several conditions. Celiac disease should be screened for with tissue transglutaminase IgA antibodies, as refractory IDA may be the presenting symptom in 5 to 6 percent of celiac patients. Autoimmune gastritis and Helicobacter pylori gastritis produce achlorhydria, which impairs the absorption of non-heme iron (which requires an acidic environment for reduction to the ferrous state). Bariatric surgery, especially Roux-en-Y gastric bypass (which bypasses the duodenum, the primary site of iron absorption), results in IDA in 20 to 50 percent of patients postoperatively. Inflammatory bowel disease involving the duodenum and prior small bowel resection are additional causes. Long-term proton pump inhibitor and H2-blocker use can modestly impair iron absorption, though the clinical significance is primarily relevant with prolonged use.

Increased Demand

Pregnancy represents the most significant physiologic increase in iron demand, requiring approximately 1,000 mg of additional iron over the course of gestation to support expanded maternal red cell mass, fetal and placental development, and anticipated blood loss at delivery. Growth during adolescence, elite athletic training (with contributions from foot-strike hemolysis, GI loss, and hepcidin elevation from exercise-related IL-6), and therapy with erythropoiesis-stimulating agents (which drive functional iron deficiency by rapidly consuming available iron for red cell production) also increase iron requirements.

Genetic Causes (Rare)

Iron-refractory iron deficiency anemia (IRIDA) is a rare autosomal recessive condition caused by mutations in TMPRSS6, which encodes matriptase-2. These mutations result in constitutively elevated hepcidin levels, rendering the patient unresponsive to oral iron supplementation. IRIDA patients respond partially to intravenous iron, which bypasses the hepcidin-mediated block on intestinal absorption. Other rare genetic causes include mutations in DMT1 and loss-of-function mutations in ferroportin.

<image>A clinical algorithm flowchart for the diagnostic workup of iron deficiency anemia in adults. Start with "IDA confirmed (Ferritin <30, TSAT <20%)." First branch: Male or postmenopausal female vs. premenopausal female. For males/postmenopausal: mandatory GI evaluation with upper endoscopy and colonoscopy. If GI workup negative, consider celiac serology, H. pylori testing, video capsule endoscopy, and urinalysis. For premenopausal female: assess menstrual history. If heavy menstrual bleeding, evaluate for gynecologic causes and consider VWD screening. If normal menses or GI symptoms, proceed to GI workup. Include a separate branch for refractory IDA: consider malabsorption workup (celiac, autoimmune gastritis), IRIDA (check hepcidin, TMPRSS6 mutations), and ongoing occult blood loss. Use a clean flowchart style with decision diamonds and rectangular process boxes.</image>

Treatment - Oral Iron

Oral Iron Formulations

Several oral iron formulations are available, each with distinct characteristics regarding elemental iron content, tolerability, and absorption. Ferrous sulfate, the most commonly prescribed form, provides 65 mg of elemental iron per 325 mg tablet and remains the standard first-line treatment, though gastrointestinal side effects including nausea, constipation, and abdominal cramping occur in 30 to 50 percent of patients. Ferrous gluconate provides 38 mg of elemental iron per 325 mg tablet and is slightly better tolerated. Ferrous fumarate delivers the highest elemental iron content at 106 mg per 325 mg tablet. Polysaccharide iron complex offers 150 mg of elemental iron per capsule with fewer gastrointestinal side effects, though absorption data are variable. Ferric maltol (Accrufer), providing 30 mg of elemental iron taken twice daily, is FDA-approved for IDA in adults including those with inflammatory bowel disease and demonstrates better tolerability than traditional ferrous salts.

Optimizing Oral Iron Absorption

A landmark study by Moretti and colleagues, published in Lancet Haematology in 2015, demonstrated that single-dose alternate-day iron administration results in greater fractional absorption than the traditional twice- or thrice-daily dosing regimen. This counterintuitive finding is explained by the hepcidin surge that occurs with each oral iron dose: acute iron absorption triggers a transient increase in hepcidin that suppresses absorption of subsequent doses for approximately 24 hours. By spacing doses to every other day, the hepcidin level returns to baseline, allowing more efficient absorption of the next dose. This finding represents a paradigm shift in oral iron therapy.

Additional strategies to optimize absorption include taking iron on an empty stomach and co-administering vitamin C (200 mg), which enhances absorption two- to three-fold by reducing ferric iron to the more readily absorbed ferrous form. Co-administration with proton pump inhibitors, calcium supplements, tea, coffee, and dairy products should be avoided, as all of these impair iron absorption. With appropriate therapy, a reticulocyte response is expected within 5 to 10 days, and hemoglobin should increase by 1 to 2 g/dL per month. Iron supplementation should be continued for 3 to 6 months after hemoglobin normalization to replenish iron stores, with a goal ferritin of 50 to 100 ng/mL or higher.

Treatment - Intravenous Iron

Indications for IV Iron

Intravenous iron is indicated when oral iron is insufficient, inappropriate, or impractical. Specific indications include oral iron intolerance or failure after an adequate trial of 4 to 6 weeks, malabsorption from celiac disease, bariatric surgery, or inflammatory bowel disease, and ongoing losses that exceed the capacity for oral replacement. In chronic kidney disease patients on dialysis, KDIGO guidelines recommend maintaining TSAT between 20 and 30 percent and ferritin between 200 and 500 ng/mL, which typically requires regular IV iron supplementation. Heart failure with iron deficiency, based on the AFFIRM-AHF and IRONMAN trials, represents an increasingly recognized indication. Perioperative settings where rapid iron repletion is needed and IRIDA, where oral iron is inherently ineffective, are additional indications.

IV Iron Formulations

The currently available IV iron formulations differ in their maximum single dose, infusion time, and adverse effect profiles. Iron sucrose (Venofer) can be administered in doses of 200 to 300 mg over 15 to 60 minutes without a test dose but requires multiple sessions for full repletion. Ferric gluconate (Ferrlecit) is primarily used in dialysis patients with a maximum single dose of 250 mg. Ferric carboxymaltose (Injectafer) allows high single doses of up to 750 mg for patients weighing 50 kg or less and 1,000 mg for those over 50 kg, administered in a convenient two-dose regimen. Ferumoxytol (Feraheme) is given in two 510 mg doses 15 minutes apart but causes MRI artifacts for weeks after infusion. Ferric derisomaltose (Monoferric) permits true single-dose total replacement of up to 1,000 mg (or 20 mg/kg) and has the lowest risk of hypophosphatemia among current formulations. Low-molecular-weight iron dextran (INFeD/CosmoFer) allows total dose infusion of over 2,000 mg in a single sitting but requires a test dose and carries the highest risk of anaphylaxis among available products.

Hypophosphatemia with IV Iron

A clinically significant adverse effect that distinguishes certain IV iron formulations is the induction of hypophosphatemia through elevation of fibroblast growth factor 23 (FGF23). This effect is most pronounced with ferric carboxymaltose, with hypophosphatemia occurring in up to 50 percent of patients. FGF23 elevation leads to renal phosphate wasting, and with repeated dosing, this can result in osteomalacia. Phosphate levels should be monitored approximately two weeks after infusion. The PHOSPHARE-IDA trial demonstrated that ferric derisomaltose has a significantly lower risk of hypophosphatemia compared to ferric carboxymaltose, making it the preferred formulation when repeated dosing is anticipated. Symptomatic hypophosphatemia may manifest as fatigue, bone pain, and muscle weakness.

Safety Considerations

The anaphylaxis rate with newer IV iron formulations is extremely low, less than 1 in 200,000 infusions, and is substantially higher with iron dextran. The Fishbane reaction, also known as complement activation-related pseudo-allergy (CARPA), presents with chest tightness, flushing, and back pain and is important to distinguish from true anaphylaxis. It is not an IgE-mediated allergic reaction and does not preclude rechallenge; the infusion can be stopped briefly and then restarted at a slower rate. Patients should not be labeled as having an "iron allergy" based on a Fishbane reaction. Following IV iron infusion, serum iron and TSAT will be falsely elevated for 24 to 48 hours, and ferritin peaks at approximately 7 to 9 days before stabilizing at 4 to 6 weeks. Iron studies should not be rechecked sooner than 4 to 6 weeks post-infusion, with 8 weeks being the preferred interval.

<image>A comparison infographic of intravenous iron formulations showing five IV iron products arranged in columns. For each product (iron sucrose, ferric carboxymaltose, ferumoxytol, ferric derisomaltose, and low-molecular-weight iron dextran), display: the brand name, maximum single dose, number of sessions needed for 1000 mg total repletion, infusion time, requirement for test dose (yes/no), relative risk of hypophosphatemia (shown as a bar graph or heat scale), and relative risk of anaphylaxis. Include a visual representation of the iron-carbohydrate complex structure for each (spherical nanoparticle with iron core and carbohydrate shell of varying thickness). Medical education poster style with clean layout.</image>

Special Populations

Iron Deficiency in Heart Failure

The role of iron replacement in heart failure has been established through a series of landmark trials. The FAIR-HF trial in 2009 demonstrated that IV ferric carboxymaltose improved symptoms, functional capacity, and quality of life in patients with heart failure with reduced ejection fraction (HFrEF) and iron deficiency. The AFFIRM-AHF trial in 2020 showed that IV ferric carboxymaltose reduced heart failure hospitalizations (rate ratio 0.74, p=0.013) in patients hospitalized for acute heart failure. The IRONMAN trial in 2022, using IV ferric derisomaltose, demonstrated a trend toward reduced cardiovascular death and heart failure hospitalization (rate ratio 0.82, p=0.07). The HEART-FID trial in 2023 showed modest benefit of IV ferric carboxymaltose on a hierarchical composite endpoint in HFrEF. Based on this evidence, the ESC 2021 guidelines provide a Class IIa recommendation for IV iron supplementation in symptomatic HFrEF patients with iron deficiency.

Iron Deficiency in CKD

The PIVOTAL trial in 2019 compared high-dose proactive IV iron sucrose (targeting ferritin 400 to 600 ng/mL) with low-dose reactive iron sucrose (triggered when ferritin fell below 200 ng/mL) in hemodialysis patients. The high-dose strategy reduced cardiovascular events and ESA requirements without increased infection risk, supporting a more aggressive approach to iron repletion in dialysis patients. Current KDIGO guidelines recommend a trial of IV iron when TSAT is 30% or below and ferritin is 500 ng/mL or below.

Iron Deficiency in IBD

Iron deficiency is highly prevalent in inflammatory bowel disease, affecting 36 to 90 percent of patients with active disease. The etiology is multifactorial, encompassing chronic blood loss, malabsorption from inflamed or resected intestinal mucosa, and inflammation-driven hepcidin elevation. IV iron is preferred over oral iron in patients with active disease, oral iron intolerance, hemoglobin below 10 g/dL, or the need for rapid repletion. European Crohn's and Colitis Organisation (ECCO) guidelines position IV iron as first-line therapy for IDA in the setting of active IBD.

Key Clinical Pearls

• Every-other-day oral iron dosing improves fractional absorption and tolerability compared to daily dosing - a paradigm shift in oral iron therapy
• Ferritin <30 ng/mL is diagnostic of iron deficiency regardless of clinical context; ferritin 30-100 ng/mL in inflammatory states requires TSAT, sTfR, or reticulocyte Hb for confirmation
• Always investigate the CAUSE of iron deficiency; in males and postmenopausal females, GI malignancy must be excluded
• Ferric carboxymaltose causes hypophosphatemia in up to 50% of patients; consider ferric derisomaltose when repeated dosing is anticipated
• The Fishbane reaction is NOT anaphylaxis; stopping the infusion briefly and restarting at a slower rate is appropriate; do not label the patient as "iron allergy"
• Recheck ferritin and TSAT no sooner than 6-8 weeks after IV iron administration

References

1. Camaschella C. Iron-Deficiency Anemia. N Engl J Med. 2015;372(19):1832-1843.
2. Auerbach M, Adamson JW. How we diagnose and treat iron deficiency anemia. Am J Hematol. 2016;91(1):31-38.
3. Moretti D, et al. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood. 2015;126(17):1981-1989.
4. Ponikowski P, et al. Ferric carboxymaltose for iron deficiency at discharge after acute heart failure (AFFIRM-AHF). Lancet. 2020;396(10266):1895-1904.
5. Macdougall IC, et al. Intravenous Iron in Patients Undergoing Maintenance Hemodialysis (PIVOTAL). N Engl J Med. 2019;380(5):447-458.

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