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.

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.

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 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 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 people with early-stage Hashimoto’s thyroiditis who are not taking thyroid medication, taking 200 micrograms of selenium yeast daily for six months is linked to a small decrease in TSH hormone levels, whereas those not taking selenium experience a small increase.