In people with Hashimoto's thyroiditis who have normal selenium levels, taking selenium supplements daily for six months or longer is linked to sustained increases in thyroid antibodies and hormone levels over five years.

In adults with Hashimoto's thyroiditis who are not taking levothyroxine, taking 200 micrograms of selenium yeast daily for six months is linked to a measurable decrease in two types of thyroid antibodies, while those not taking selenium show no change.

In people with Hashimoto's thyroiditis, lower levels of selenium in the blood are linked to higher levels of antibodies that attack the thyroid gland, even when overall selenium levels are not deficient compared to healthy individuals.

Among adults with Hashimoto's thyroiditis who have normal selenium levels, taking selenium supplements has been linked to a higher chance of developing other autoimmune conditions, including Sjögren's syndrome, over five years.

In people with Hashimoto’s thyroiditis, taking selenium supplements raises levels of selenium and certain antioxidant proteins in the blood, and these changes are linked to lower levels of thyroid autoantibodies and TSH.

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.