Is hypothyroidism related to macrocytic anemia?

From the Guidelines

Hypothyroidism is indeed related to macrocytic anemia, as evidenced by its inclusion in the classification of macrocytic anemia causes. According to the European consensus on the diagnosis and management of iron deficiency and anaemia in inflammatory bowel diseases 1, hypothyroidism is listed as a potential cause of macrocytic anemia with normal or low reticulocytes. This connection is thought to occur due to the role of thyroid hormones in red blood cell production and maturation.

Some key points to consider in the relationship between hypothyroidism and macrocytic anemia include:

• Thyroid hormones play a crucial role in erythropoiesis, and their deficiency can lead to impaired red blood cell production and maturation.
• Hypothyroidism can cause decreased absorption of essential nutrients like vitamin B12 and folate, which are vital for proper red blood cell development.
• The macrocytosis in hypothyroid patients may occur even without B12 or folate deficiency, suggesting a direct effect of thyroid hormone deficiency on erythropoiesis.
• Correcting thyroid dysfunction with appropriate thyroid hormone replacement, typically levothyroxine at individualized doses, often resolves the anemia without additional treatment 1.

It is essential to check vitamin B12 and folate levels in patients with hypothyroidism and macrocytic anemia to rule out concurrent deficiencies that might require specific supplementation. The primary approach to treating a patient with both hypothyroidism and macrocytic anemia is to correct the thyroid dysfunction, which can lead to the resolution of the anemia.

From the Research

Relationship Between Hypothyroidism and Macrocitic Anemia

• Hypothyroidism can cause a wide variety of anemic disorders, including macrocytic anemia 2, 3, 4.
• Macrocytic anemia in hypothyroidism may result from malabsorption of vitamin B12, folic acid, or pernicious anemia 2.
• Pernicious anemia occurs 20 times more frequently in patients with hypothyroidism than in the general population 2.
• Macrocytosis is found in up to 55% of patients with hypothyroidism and may result from the insufficiency of thyroid hormones themselves without a nutritive deficit 2.
• A case study reported a patient with pernicious anemia who developed macrocytic anemia after many years of monthly vitamin B12 therapy, which was later found to be caused by primary hypothyroidism 4.
• Treatment with levothyroxine led to correction of the hematologic abnormalities over a four-month period in this patient 4.

Prevalence of Macrocitic Anemia in Hypothyroidism

• Anemia prevalence was 43% in the overt hypothyroid group and 39% in the subclinical hypothyroid group, with no difference between the hypothyroid groups in terms of anemia 3.
• Macrocytic or microcytic anemia occurs less frequently than normocytic anemia in hypothyroidism 5.
• A study found that 57.1% of patients with hypothyroidism had anemia, with normocytic or macrocytic anemia present in 46.3% of cases 6.

Mechanisms and Therapy

• Thyroid hormones stimulate the proliferation of erythrocyte precursors both directly and via erythropoietin production enhancement 5.
• Anemia in hypothyroidism might result from bone marrow depression, decreased erythropoietin production, comorbid diseases, or concomitant iron, vitamin B12, or folate deficiency 5.
• Restoration of euthyroid state can correct most erythrocyte abnormalities, including macrocytic anemia 6.