Hypothyroidism causes macrocytosis.

Hypothyroidism is a well-established clinical cause of macrocytosis, characterized by an elevated mean corpuscular volume (MCV). It represents a primary etiology of non-megaloblastic macrocytosis, occurring independently of nutritional status.

Clinical and effectiveness evidence

• Established Association: Clinical studies consistently show that patients with untreated primary or subclinical hypothyroidism have significantly higher MCV levels compared to euthyroid individuals. Data from large-scale cross-sectional studies, such as NHANES, demonstrate a negative correlation between free T3/T4 levels and MCV, indicating that as thyroid function decreases, red blood cell size tends to increase.
• Resolution with Treatment: Macrocytosis in these patients is often reversible. Case reports have documented elevated MCV that failed to respond to Vitamin B12 or folate supplementation but normalized following levothyroxine replacement therapy once a euthyroid state was achieved.
• Prevalence: Hypothyroidism is identified in approximately 10–20% of non-megaloblastic macrocytosis cases. In clinical practice, it is ranked as a top-tier differential diagnosis when B12 and folate levels are within normal ranges.

Mechanistic explanations

• Transcriptional Regulation: Thyroid hormones (specifically T3) are direct regulators of erythropoiesis. They bind to thyroid hormone receptors (TRα and TRβ) to activate the transcription of GATA-1, a master erythroid regulator. This pathway is essential for the activation of downstream genes like Klf1 and beta-globin, which govern hemoglobin synthesis and cell maturation.
• Disrupted Differentiation: In hypothyroid states, the lack of T3-bound TRα activity impairs GATA-1 activation, leading to a reduction in bone marrow erythrocytic progenitors and slower terminal differentiation. This results in the presence of larger, immature erythroblasts and "round" macrocytes rather than the "oval" macrocytes seen in B12/folate deficiency.
• Secondary Factors: Hypothyroidism also reduces red blood cell production efficiency by lowering erythropoietin (EPO) levels through decreased renal stimulation and compromising the bone marrow microenvironment.

Diagnostic considerations

• Morphological Distinction: On a peripheral smear, hypothyroidism typically presents as non-megaloblastic macrocytosis (round macrocytes without hypersegmented neutrophils). This contrasts with the megaloblastic changes (oval macrocytes and hypersegmented neutrophils) characteristic of Vitamin B12 or folate deficiency.
• Standard Protocols: Diagnostic algorithms for unexplained macrocytosis recommend screening with Thyroid-Stimulating Hormone (TSH) tests, especially when the MCV is mildly elevated (typically between 100–110 fL).

Bottom line

Hypothyroidism is a recognized cause of non-vitamin-related macrocytosis. It results from reduced erythropoietic efficiency and impaired GATA-1-mediated differentiation, leading to larger red blood cell indices that typically normalize with thyroid hormone replacement therapy.