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.

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.

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.

Taking 200 micrograms of selenium daily for a year raises selenium levels in the blood of adults with autoimmune hypothyroidism who are on levothyroxine, but it does not change how efficiently the body converts thyroxine into triiodothyronine.

In adults with Graves' disease taking antithyroid medication, taking selenium supplements at 100–300 μg per day for 3 to 6 months lowers levels of FT3 and FT4 hormones in the blood, but these reductions do not remain after 9 months of treatment.

Taking 200 micrograms of selenium daily for a year does not improve how people with autoimmune hypothyroidism feel on a standardized quality-of-life survey, even though their thyroid antibody levels decrease.