An essential trace element, selenium is nonmetallic, gray in appearance, and similar to sulfur in its chemical composition. It is often available in single or multivitamin supplements.
Selenium is needed to activate a number of hormones produced by the thyroid gland.
Selenium also plays a vital role in the functioning of the immune system. Studies have found that selenium supplementation stimulates the activity of white blood cells. It also enhances the effect of vitamin E, one of three vitamins that act as antioxidants. Brazil nuts are the best source of selenium. Yeast, whole grains, garlic and seafood are also good sources.