Hypothyroidism Increases MCV While Decreasing Hemoglobin
The answer is b. hypothyroidism—this condition characteristically produces macrocytic anemia (elevated MCV) with decreased hemoglobin levels. 1, 2
Why Hypothyroidism Causes This Pattern
Hypothyroidism directly causes macrocytic anemia independent of nutritional deficiencies. In a series of 202 hypothyroid patients, the average MCV was 90 fL, with 29 of 53 patients (who had normal B12, folate, and iron levels) showing MCV >90 fL, and three patients exceeding 100 fL. 1 Importantly, 9 of these 53 patients had both anemia and elevated MCV—the classic macrocytic anemia of hypothyroidism—despite having completely normal vitamin levels. 1
Mechanism of Thyroid-Related Macrocytosis
- Thyroid hormones directly regulate erythropoiesis. FT4 levels correlate inversely with MCV and MCH, meaning lower thyroid hormone levels produce larger red blood cells. 2
- The macrocytosis resolves with thyroid replacement alone. MCV invariably falls with thyroxine treatment, even when initial values are within normal range, proving this is a direct thyroid effect rather than a nutritional deficiency. 1
- Anemia prevalence in hypothyroidism is 57.1%, with normocytic or macrocytic anemia present in 46.3% of these cases. 2
Treatment Response Confirms the Diagnosis
- Hemoglobin rises with thyroxine alone in hypothyroid patients who have normal B12, folate, and iron levels, demonstrating that the anemia is directly caused by thyroid deficiency. 1
- Complete correction occurs after restoration of euthyroid state for at least 3 months, with normalization of both MCV and hemoglobin. 2
Why the Other Options Are Incorrect
Pregnancy (Option a)
Pregnancy causes dilutional anemia with normal or slightly decreased MCV, not elevated MCV. The WHO defines anemia in pregnancy as hemoglobin <110 g/L in the second and third trimester. 3 Pregnancy-related anemia is typically normocytic or microcytic (from iron deficiency), not macrocytic.
Anemia of Chronic Disease (Option c)
Anemia of chronic disease produces normal or low MCV, never elevated MCV. 3 The European consensus explicitly states that in ACD, MCV and MCH "may be normal or low" but macrocytosis indicates B12 or folate deficiency, not ACD itself. 3 Microcytosis in ACD indicates iron-restricted anemia (functional iron deficiency), while normocytosis with normal or elevated ferritin is the classic ACD pattern. 3
Critical Clinical Pitfall
Do not assume macrocytic anemia in hypothyroidism is always due to pernicious anemia. The incidence of concurrent pernicious anemia in hypothyroid patients is 8.5%, but the MCV of hypothyroid patients with low B12 is often no greater than in patients with uncomplicated hypothyroidism. 1 MCV is not a useful discriminant for diagnosing pernicious anemia in hypothyroidism—you must measure B12 levels directly. 1
When Hypothyroidism Coexists with B12 Deficiency
- Always exclude B12 deficiency before attributing macrocytosis solely to hypothyroidism, as both conditions can coexist and B12 deficiency requires specific treatment to prevent irreversible neurological damage. 4, 5
- The macrocytosis may not be more pronounced when both conditions are present, making laboratory confirmation essential. 1