People with Hashimoto’s thyroiditis tend to have lower levels of selenium in their blood than what is needed for optimal function of selenium-dependent proteins, even though their levels are not statistically different from healthy individuals, suggesting that group comparisons may miss individual insufficiency.

Among adults with Hashimoto's thyroiditis who have normal selenium levels, taking selenium supplements may be linked to a 63% higher chance of dying from any cause over five years, even though there is no evidence it increases the risk of thyroid cancer or the need for thyroid surgery.

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.

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.

Taking 200 micrograms of selenium yeast daily for six months raises selenium levels in the blood and increases the activity of two selenium-dependent proteins involved in antioxidant defense in adults with Hashimoto’s thyroiditis.

For adults with Hashimoto's thyroiditis who have normal selenium levels, taking selenium supplements does not lower thyroid-stimulating hormone (TSH) or stop the condition from worsening to overt hypothyroidism, and may cause TSH levels to rise over time.