Selenium is a chemical element with the atomic number 34, represented by the chemical symbol Se. It is toxic in large amounts, but trace amounts of it are necessary for cellular function in most animals, forming the active centre of the enzymes like glutathione peroxidase, thioredoxin reductase and three known deiodinase enzymes. Natural sources of selenium are water (less than 10 μg/l) and selenium that has been bio concentrated by plants and animals (from 0 to 1 mg/Kg). The selenium normal dietary intake (food and drink water) ranges from 30 to 130 μg/die and the optimum intake estimate ranges from 50 to 70 μg a day. Consequently the dietary supplement, typically 50 to 200 μg/die for adult, can be dangerous and, in Italy, the estimated greatest dietary supplement should be less than 83 μg/die for adult (105 μg a day for adult in the USA, according to the EPA).