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 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.

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.

Taking selenium supplements may temporarily lower levels of certain thyroid antibodies in some people, but these changes are not consistent and do not last over time, regardless of whether the person is also taking levothyroxine.