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.

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.

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.

Giving children with congenital hypothyroidism selenium supplements at 20–60 mcg per day for three months does not change the levels of thyroid hormones T4 and T3 in their blood, or the ratio between them.