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 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.

In people with low selenium levels from limited food intake or nutritional support, a specific blood ratio of thyroid hormones (free T4 to free T3) is higher than in people with normal selenium levels, which may help detect selenium deficiency before symptoms appear.

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.

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.

People with autoimmune thyroid disease tend to have lower levels of selenium in their blood, whether their thyroid is underactive or overactive. This suggests the low selenium levels are linked to the autoimmune condition itself, not to whether the thyroid is producing too much or too little hormone.