Causes of Microcytic Hypochromic Anemia
Microcytic hypochromic anemias are primarily caused by nutritional iron deficiency, iron loss from gastrointestinal disease, iron malabsorption, hemoglobinopathies (including thalassemia syndromes), anemia of chronic disease, and genetic disorders of iron metabolism or heme synthesis 1.
Common Acquired Causes
The most frequent causes in clinical practice include:
- Iron deficiency anemia - the single most common cause, resulting from nutritional deficiency, blood loss, or malabsorption 2
- Anemia of chronic disease - presents with low serum iron despite adequate or increased iron stores 1
- Lead toxicity - causes impaired heme synthesis 3
Hemoglobinopathies
Thalassemia syndromes should be strongly considered in patients with particularly low MCV in the absence of systemic iron deficiency 2. Beta-thalassemia trait is characterized by elevated HbA2 levels (mean 5.2% ± 0.8%), significantly lower MCV and MCH values, and higher RBC counts 4. These patients often present with microcytic anemia that mimics iron deficiency but does not respond to iron supplementation.
Genetic Disorders of Iron Metabolism
When ferritin and/or transferrin saturation are elevated, or when TSAT is low with low-normal ferritin levels (>20 mg/L), consider genetic disorders 1:
Iron Metabolism Defects:
- Iron refractory iron deficiency anemia (IRIDA) - due to TMPRSS6 defects, presents with anemia refractory to oral iron 1
- Hypotransferrinemia - due to TF defects, causes microcytic anemia with low serum iron but high ferritin 1
- DMT1 deficiency - due to SLC11A2 defects, presents with microcytic anemia and systemic iron loading 1
Sideroblastic Anemias (Heme Synthesis Defects):
These disorders are critical to identify because iron overload causes greater morbidity and mortality than the anemia itself 1.
X-linked sideroblastic anemia (XLSA) - due to ALAS2 defects, the most common genetic sideroblastic anemia. Presents with mild hypochromic microcytic anemia and systemic iron overload, often in first two decades of life 1. Unrecognized tissue iron loading can lead to severe morbidity and mortality 1.
SLC25A38 deficiency - presents with severe, transfusion-dependent microcytic hypochromic anemia in childhood, clinically similar to thalassemia major, with ring sideroblasts on bone marrow 1
ABCB7 defects - causes XLSA with ataxia, presenting with mild slightly microcytic sideroblastic anemia plus cerebellar ataxia 1
STEAP3 deficiency - ferroreductase defect causing severe microcytic anemia 1
GLRX5 defects - causes microcytic sideroblastic anemia with iron loading 1
Porphyrias
Erythropoietic protoporphyria (EPP) - presents with mild normocytic to microcytic anemia, low-to-normal ferritin, and painful photosensitivity in childhood 1
Congenital erythropoietic porphyria (CEP) - presents with severe microcytic hypochromic hemolytic anemia combined with painful cutaneous photosensitivity 1
Critical Diagnostic Pitfalls
A common pitfall is presuming all microcytic hypochromic anemia represents iron deficiency at initial presentation 5. Failure to respond to iron supplementation and iron indices inconsistent with IDA (such as elevated ferritin or transferrin saturation) should prompt immediate diagnostic reassessment 5.
In older patients with iron-refractory microcytic anemia and discordant iron indices, bone marrow Perls staining is critical for identifying ring sideroblasts 5. This distinguishes sideroblastic anemias from iron deficiency and prevents harmful continued iron supplementation that worsens iron overload.
Family history of anemia refractory to iron supplementation, neurologic disease, or skin photosensitivity are red flags for genetic disorders 1.