Selenium is a trace mineral, which means that the body only needs a small amount of it. It is found naturally in foods or as a supplement. Selenium is an essential component of various enzymes and proteins, called selenoproteins, that help to make DNA and protect against cell damage and infections; these proteins are also involved in reproduction and the metabolism of thyroid hormones. Most selenium in the body is stored in muscle tissue, although the thyroid gland holds the highest concentration of selenium due to various selenoproteins that assist with thyroid function.