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Why hypothyroidism causes anaemia

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hypothyroidism AND anemia

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hypothyroidism anemia mechanisms review

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https://www.ncbi.nlm.nih.gov/books/NBK459295

Hypothyroidism causes anemia mainly because thyroid hormone normally supports red blood cell production.
  1. Reduced erythropoietin (EPO)
    • Hypothyroidism lowers basal metabolic rate and tissue oxygen demand.
    • The kidneys therefore release less EPO, the hormone that stimulates bone marrow erythropoiesis.
  2. Direct reduction in marrow activity
    • Thyroid hormone helps erythroid precursor cells proliferate. Its deficiency makes the marrow hypoproliferative, so fewer reticulocytes and red cells are produced.
This produces the typical normocytic, normochromic, low-reticulocyte anemia, usually mild to moderate. It is sometimes called an adaptive anemia because red-cell mass falls along with the body's oxygen requirement. Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 685.
Other associated mechanisms can change the MCV:
  • Microcytic anemia: iron deficiency due to menorrhagia in women, and sometimes reduced gastrointestinal iron absorption.
  • Macrocytic anemia: autoimmune hypothyroidism can coexist with autoimmune gastritis/pernicious anemia, causing vitamin B12 deficiency. Pernicious anemia is reported much more often in people with hypothyroidism. Frameworks for Internal Medicine, p. 334.
So, a useful summary is:
↓ T3/T4 → ↓ metabolism and oxygen need → ↓ renal EPO + ↓ marrow erythropoiesis → anemia.
In a hypothyroid person with anemia, assess CBC with indices, reticulocyte count, ferritin/iron studies, B12 and folate, and consider menstrual or gastrointestinal blood loss. The NIH review of macrocytic anemia also notes the combination of reduced marrow erythroid proliferation, lower EPO, and possible iron/B12/folate deficiencies.
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