Selenium-dependent enzymes and selenoprotein P regulate immune and endothelial cell function. (PMID: 16607122). Thyroid hormone synthesis, metabolism and action require adequate availability of the essential trace elements iodine and selenium, which affect homeostasis of thyroid hormone-dependent metabolic pathways. The three selenocysteine-containing iodothyronine deiodinases constitute a novel gene family. Selenium is retained and deiodinase expression is maintained at almost normal levels in the thyroid gland, the brain and several other endocrine tissues during selenium deficiency, thus guaranteeing adequate local and systemic levels of the active thyroid hormone T(3). (PMID: 16131327). The trace element nutrient selenium (Se) discharges its well-known nutritional antioxidant activity through the Se-dependent glutathione peroxidases. It also regulates nuclear factor activities by redox mechanisms through the selenoprotein thioredoxin reductases. Converging data from epidemiological, ecological, and clinical studies have shown that Se can decrease the risk for some types of human cancers, especially those of the prostate, lung, and colon. Mechanistic studies have indicated that the methylselenol metabolite pool has many desirable attributes of chemoprevention, targeting both cancer cells and vascular endothelial cells, whereas the hydrogen selenide pool in excess of selenoprotein synthesis can lead to DNA single strand breaks, which may be mediated by some reactive oxygen species. (PMID: 16356132). SePP (selenoprotein P) is the major transporter of Se in the serum. Moreover, in the sanctuary area of the brain, SePP was shown to play a hitherto unexpected role as a local Se storage and recycling protein that directly maintains brain Se levels. Physiologically, it exists as an ion in the body. The function of Se is important in normal brain metabolism, redox regulation, antioxidant defenses, thyroid hormone metabolism and the development of neurodegenerative conditions. (PMID: 15720294). In areas where soils are low in bioavailable selenium (Se), potential Se deficiencies cause health risks for humans. (PMID: 16028492) Dietary selenium comes from cereals, meat, fish, and eggs. The recommended dietary allowance for adults is 55 micrograms per day.