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 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.

Taking selenium supplements daily for a year does not lower the amount of levothyroxine that adults with autoimmune hypothyroidism need to take to manage their thyroid hormone levels.

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.

Taking selenium supplements may lower levels of thyroid peroxidase antibodies in adults with chronic autoimmune thyroiditis who are already taking levothyroxine, with the greatest reduction seen at six months. However, it does not change thyroid-stimulating hormone levels or improve symptoms related to the condition.

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.