Anemia due to hypothyroid

Even subclinical hypothyroid (reduced systemic thyroid effect despite normal lab results) has far-reaching consequences because the ability of every cell in the body to do its work is diminished. A study just published in the European Journal of Internal Medicine further confirms that hypothyroid can be a cause of anemia. The authors observe:

"Hypothyroidism is associated with normocytic anaemia. Indeed, a limited number of studies have shown significant associations between free thyroxin (T4) and erythrocyte indices. These studies did not include vitamin B12, folic acid, iron and renal function in the analyses. We therefore studied the association between thyroid hormones and erythrocyte indices in a population-based cohort of older euthyroid subjects, with adjustment for major confounding parameters."

In other words, anemia with the normal size of red blood cells (normocytic) has been observed to be associated with low thyroid hormone (free thyroxine/T4), but up until the authors' investigation this observation had not been dissociated from other common factors that can cause anemia. So they used data from the the Longitudinal Aging Study Amsterdam (LASA) ongoing in the Netherlands to analyze associations between free T4, thyroid stimulating hormone (TSH) and erythrocyte indices (hemoglobin content, hematocrit, mean cell volume (MCV) and erythrocyte count) in subjects whose thyroid labs were normal, while taking into consideration vitamin B12, folic acid, iron levels and renal function. The data showed a significant association of anemia with T4:

"In 708 euthyroid older subjects, an increase of 5pmol/L free T4 was associated with a mean increase of 0.12mmol/L or 0.19g/dL of haemoglobin, 0.068 1012/L erythrocytes and 0.006L/L haematocrit, respectively). Free T4 was not significantly associated with MCV. TSH appeared not to be associated with any of the erythrocyte indices."

As the authors conclude, this tells us that the production of red blood cells (erythropoiesis) is influenced by the stimulation of thyroid hormone even within the range of relatively normal thyroid values:

"In a cohort of older subjects, free T4, but not TSH, was associated with erythrocyte indices, confirming the role of thyroid hormones in the regulation of erythropoiesis."

Especially in the presence of chronic inflammation, clinicians should remember to consider  hypothyroid, subclinical or otherwise, as a potential cause of anemia.