Selenium is a trace mineral your body needs for proper functioning of the thyroid gland and immune system. It is an antioxidant that protects cells and tissues against free radical damage. The best food sources of selenium are Brazil nuts, seafood, organ meats, meat, and eggs. Whole grains like brown rice and cereals also contain the mineral. Selenium deficiency is rare. It may occur in regions where selenium content in the soil is low, especially in those who are vegetarian or vegan. Selenium levels may also be low in those who suffer from HIV and in those who undergo long-term kidney dialysis. Dialysis removes some selenium from the blood.