Glutathione peroxidase (GSH-Px) was the first selenium metalloenzyme identified.Glutathione peroxidase catalyzes the reduction of hydrogen peroxide and lipid h yp r o p e r o x id e , p r e v e n t in g o x id a ti v e d a ma g e t o b o d y t i ss u e s . A se c o n d selenometalloenzyme, iodothyronine 5' - deiodinase, also was identified. This enzyme catalyzes the deiodination of thyroxine (T 4) to the more metabolically active triiodothyronine (T3) in tissues. Factors affecting selenium requirements are not well-defined. Because the function of vitamin E and selenium are inter-related; a diet lowin vitamin E may increase the amount of selenium needed to prevent certain abnormalities such as nutritional muscular dystrophy /white muscle disease. High dietary sulphur has resulted in an increased incidence of white muscle disease in some, but not all, studies. Selenium toxicity is known as blind staggers/alkali disease and degnala.