Selenium (Se) is a trace mineral that is essential to human health. Selenium, in the form of selenoproteins, carries out various functions in normal health and metabolism. In particular, glutathione peroxidase (G-Px), a selenoprotein, performs antioxidant activities that protect against reactive oxygen and nitrogen species. Iodothyronine deiodinases that convert inactive thyroxine (T4) to active thyroid hormone, triiodothyronine (T3), are selenium-dependent. Selenium plays a role in the immune system functioning and the progression of HIV to AIDS. Selenium deficiency has been implicated in cardiovascular disease, infertility, myodegenerative diseases, and cognitive decline. The role of selenium in cancer treatment is currently being studied. In the 200 years since its discovery in 1817, the role of selenium in human health has evolved. Previously maligned as a carcinogen, it is now being understood as a vital nutrient, albeit one with a low window from being therapeutic to toxic.
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