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 selenium supplements at 100–300 micrograms per day for 3 to 6 months may lead to a small rise in thyroid-stimulating hormone levels in adults with Graves' disease who are on antithyroid drugs, but this change does not continue after 6 months.

In people with Graves' disease taking antithyroid medications, adding selenium may lead to faster normalization of thyroid hormone levels at six months, but this effect does not last until nine months and differs depending on the type and amount of selenium used.

Taking selenium supplements daily at 100–300 micrograms for six months may lower levels of antibodies targeting the thyroid-stimulating hormone receptor in adults with Graves' disease who are on antithyroid drugs, but this reduction does not last beyond nine months.

Taking selenium supplements does not keep thyroid hormone levels improved in people with Graves' disease beyond six months; after nine months, these levels return to what they were before treatment, even if supplementation continues.

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.