The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchins, fish, and chickens. Selenium enzymes are involved in the small reducing molecules glutathione and thioredoxin. One family of selenium-bearing molecules (the glutathione peroxidases) destroys peroxide and repairs damaged peroxidized cell membranes, using glutathione. Another selenium-bearing enzyme in some plants and in animals (thioredoxin reductase) generates reduced thioredoxin, a dithiol that serves as an electron source for peroxidases and also the important reducing enzyme ribonucleotide reductase that makes DNA precursors from RNA precursors.Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium, vanadium, magnesium, copper, and zinc, may have been lacking in some terrestrial mineral-deficient areas. Marine organisms retained and sometimes expanded their selenoproteomes, whereas the selenoproteomes of some terrestrial organisms were reduced or completely lost. These findings suggest that, with the exception of vertebrates, aquatic life supports selenium use, whereas terrestrial habitats lead to reduced use of this trace element. Marine fishes and vertebrate thyroid glands have the highest concentration of selenium and iodine. From about 500 million years ago, freshwater and terrestrial plants slowly optimized the production of "new" endogenous antioxidants such as ascorbic acid (vitamin C), polyphenols (including flavonoids), tocopherols, etc. A few of these appeared more recently, in the last 50–200 million years, in fruits and flowers of angiosperm plants. In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic period.The deiodinase isoenzymes constitute another family of eukaryotic selenoproteins with identified enzyme function.
Selenium is a cofactor for the three thyroid hormone deiodinases, helping activate and then deactivate various thyroid hormones and their metabolites. Isolated selenium deficiency is now being investigated for its role in induction autoimmune reactions in thyroid gland in Hashimotos disease. However, in a case of combined iodine and selenium deficiency, selenium deficiency was shown to play a thyroid-protecting role. See also
Abundance of elements in Earths crust
ACES (nutritional supplement)
Selenium yeast

Notes
References
External links

Selenium at The Periodic Table of Videos (University of Nottingham)
National Institutes of Health page on Selenium
Assay Archived 2012-02-26 at the Wayback Machine
ATSDR – Toxicological Profile: Selenium
CDC – NIOSH Pocket Guide to Chemical Hazards
Peter van der Krogt elements site