Although selenium is required in very tiny amounts (micrograms, i.e. millionths of a gram), and, like other trace minerals, can be toxic in larger doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral, sometimes in toxic proportions. (Some plants may accumulate selenium as a defense against being eaten by animals, but other plants such as locoweed require selenium, and their growth indicates the presence of selenium in soil).
Selenium is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that functions as cofactor for reduction of antioxidant enzymes, such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).
Selenium also plays a role in the functioning of the thyroid gland and in every cell that uses thyroid hormone, by participating as a cofactor for the three of the four known types of thyroid hormone deiodinases, which activate and then deactivate various thyroid hormones and their metabolites: the iodothyronine deiodinases are the subfamily of deiodinase enzymes that use selenium as the otherwise rare amino acid selenocysteine. (Only the deiodinase iodotyrosine deiodinase, which works on the last break-down products of thyroid hormone, does not use selenium).
Selenium shows evidence of reducing the effects of mercury toxicity.
. ^ Linus Pauling Institute at Oregon State University lpi.oregonstate.edu
. ^ "Selenium". Linus Pauling Institute at Oregon State University. Retrieved 2009-01-05.
. ^ Mazokopakis, EE; Papadakis, JA; Papadomanolaki, MG; Batistakis, AG; Giannakopoulos, TG; Protopapadakis, EE; Ganotakis, ES (2007). "Effects of 12 months treatment with L-selenomethionine on serum anti-TPO Levels in Patients with Hashimoto's thyroiditis". Thyroid : official journal of the American Thyroid Association 17 (7): 609.612. doi:10.1089/thy.2007.0040. PMID 17696828.
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