In people with low selenium levels, the ratio of two thyroid hormones (FT4 to FT3) tends to rise more reliably than the individual levels of those hormones, making it a better marker for detecting disruptions in how the body processes thyroid hormones than measuring TSH, FT4, or FT3 by themselves.

In people with low selenium levels, taking selenium supplements lowers free T4 and the ratio of free T4 to free T3 while raising free T3 levels, suggesting that selenium plays a role in how the body converts thyroid hormones and that this process can improve with supplementation.

People with low selenium levels have lower amounts of free T3 hormone in their blood compared to those with normal thyroid function, but their free T4 and TSH levels do not consistently rise, suggesting the issue is not with the thyroid gland itself but with how the body converts thyroid hormones in peripheral tissues.

In people with very low selenium levels, lower selenium amounts are linked to a higher ratio of inactive thyroid hormone (FT4) to active thyroid hormone (FT3), indicating that selenium deficiency may reduce the body’s ability to convert FT4 into FT3.

After thyroid removal, some patients have high TSH and high FT4 but low FT3, which suggests their bodies are not efficiently converting the available thyroid hormone (T4) into its active form (T3).

Taking selenium supplements does not change the levels of thyroid-stimulating hormone (TSH) in adults with chronic autoimmune thyroiditis, whether or not they are taking levothyroxine.