Children born with an underactive thyroid who take levothyroxine medication show different levels of certain thyroid-related substances in their blood compared to children with normal thyroid function, including lower selenium, T3, and thyroglobulin, and higher TSH, reverse T3, and T4.

In children with congenital hypothyroidism who are already taking levothyroxine, taking a selenium supplement for three months lowers thyroglobulin levels by about 74%, but does not change the amounts of thyroid hormones in the blood or the ratio between T4 and T3.

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.

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 autoimmune hypothyroidism taking levothyroxine, taking 200 micrograms of selenium daily for a year may lower levels of thyroid peroxidase antibodies by about 19% compared to a placebo, but it does not change the required dose of thyroid hormone or the balance between free triiodothyronine and free thyroxine.

Most cases of hypothyroidism occur because the immune system attacks the thyroid gland, and the thyroid naturally holds more selenium than most other tissues in the body.