Selenium (Se) is a constituent of glutathione peroxidase, an antioxidant enzyme that breaks down hydrogen peroxide, phospholipid hydroperoxides and other free hydroperoxides. There are four selenium atoms in glutathione peroxidase. There are three types of iodothyronine deiodinase. It removes iodine from thyroid hormone molecules, activates T4 and inactivates T3. Type I in the cells of the liver, kidney and thyroid converts T3 from T4. Type II is found in the brain, brown fat, pituitary and converts T4 to T3 and regulates TSH secretion, Type III inactivates T3. Se in food is related to the soil selenium. Keshan disease in China (a selenium deficiency resulting from low Se in the soil in Keshan, China) results in an enlarged heart and congestive heart failure. Kashin-Beck is an endemic osteoarthritis caused by selenium deficiency. Se deficiency in male and female rats has resulted in infertility. Chronic selenosis (selenium toxicity) results in loss of hair and nails, skin lesions, tooth decay & nervous system disorders. Acute Se poisoning results in nausea, vomiting, hair loss, nail changes, irritability, fatigue and neuropathy. Good sources of Selenium are foods grown in Selenium rich soil, tuna, seafoods, meats and whole grains.