Higher thyroid hormone levels went with less anemia in Brazilian adults

Higher free thyroxine levels were linked to lower odds of anemia, while TSH levels were not, in an analysis of 13,247 Brazilian adults in the ELSA-Brasil study, published Sept. 28, 2026, in Revista da Associação Médica Brasileira. Thyroid health and blood health may be connected.

The thyroid gland makes hormones that help regulate energy use throughout the body, including the bone marrow, where red blood cells are made. Anemia, too few red blood cells or too little hemoglobin, causes tiredness and shortness of breath.

What did the study do?

Researchers analyzed 13,247 participants of the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil), civil servants aged 35 to 74 in six Brazilian cities, examined between 2008 and 2010. They measured thyroid-stimulating hormone (TSH), the brain’s signal to the thyroid, and free thyroxine, the main hormone the thyroid releases. Anemia was defined as hemoglobin below 12 g/dL in women and 13 g/dL in men.

What did the study find about thyroid health?

Higher free thyroxine was linked to lower odds of anemia, compared with the lowest fifth of levels:

Free thyroxine level	Odds ratio for anemia
Second fifth	0.80
Third fifth	0.55
Fourth fifth	0.49
Fifth fifth (highest)	0.52

The association held in both men and women. TSH levels were not linked to anemia.

Why does this matter?

Low thyroid function is a known but sometimes overlooked cause of anemia. These results suggest free thyroxine levels, even within normal ranges, may relate to red blood cell production, which could prompt doctors to consider thyroid function when evaluating anemia.

How solid is this evidence?

It’s moderate. Mongevity rates it C on our A-to-E scale: a large, well-characterized cohort, but this was a cross-sectional analysis at one time point, so it can’t show which comes first. The study population, civil servants, may not represent everyone.

What are the open questions?

Longitudinal studies are needed to see whether changes in thyroid hormone predict anemia, and whether treating low thyroid improves anemia in people with mildly low levels.

What does this mean for you?

If you have unexplained tiredness or anemia, ask your doctor whether a thyroid test makes sense along with iron and vitamin B12. If you take thyroid medicine, keep regular checks of your levels.

How do I explain this at dinner?

The thyroid helps power the body, including the factory that makes red blood cells. In a big study of Brazilian adults, people with higher thyroid hormone levels were less likely to be anemic. A good question for the table: what causes of tiredness do people overlook?

In one sentence: higher free thyroxine was linked to lower odds of anemia in Brazilian adults, while TSH was not, suggesting a role for thyroid function in red blood cell health.