Selenium by Schiff/Bio Foods. Selenium is a trace mineral that is part of an antioxidant enzyme called glutathione peroxidase. It is also necessary for normal growth and proper utilization of the trace element, iodine, for normal thyroid function. Selenium also supports the antioxidant effect of vitamin E. The best dietary sources of selenium include nuts, unrefined grains, brown rice, wheat germ, and seafood.