Selenium is another mineral needed to maintain good health. The body uses selenium to make antioxidant proteins that protect cells and tissue from damage caused by free radicals, byproducts formed when cells use oxygen. Selenium also plays an important role in controlling the metabolism by taking part in regulating the production of the thyroid hormones T3 and T4. Selenium supports a vigorous immune response to infection by helping to promote the formation of white blood cells. When one gets vaccinated, selenium helps to insure that the vaccine works well to protect him from disease. Selenium augments the response to the vaccine because it increases the production of interferon, a protein produced by the body when it is invaded by a virus such as that contained in a vaccine. Interferon prevents the growth of the virus. The recommended dietary allowance of selenium is the same for men and women. After age 14 and throughout adulthood, the body needs 55 micrograms. The only time the recommendation goes up is for pregnant women. During pregnancy 60 micrograms are needed, which increases to 70 micrograms while breastfeeding. A selenium deficiency is very rare. However, a deficiency in children may show up as Keshan disease, a condition characterized by enlargement of the heart and a degrading of the heart's ability to pump blood. On the other hand, selenium is toxic at high doses. Chronic elevated selenium levels, a condition known as selenosis, leads to brittle hair and nails. An overdose also leads to severe gastrointestinal upset, skin rashes, bad breath, extreme exhaustion and problems with the central nervous system.