damage by H2O2 produced for thyroid-hormone biosynthesis. About 200 million years ago, new selenoproteins were developed as mammalian GSH-Px enzymes. Nutritional sources of selenium Dietary selenium comes from meat, nuts, cereals and mushrooms. Brazil nuts are the richest dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs). The US Recommended Dietary Allowance (RDA) of selenium for teenagers and adults is 55 µg/day. Selenium as a dietary supplement is available in many forms, including multi-vitamins/mineral supplements, which typically contain 55 or 70 µg/serving. Selenium-specific supplements typically contain either 100 or 200 µg/serving. In June 2015, the US Food and Drug Administration (FDA) published its final rule establishing the requirement of minimum and maximum levels of selenium in infant formula. The selenium content in the human body is believed to be in the 13–20 mg range. Indicator plant species Certain species of plants are considered indicators of high selenium content of the soil because they require high levels of selenium to thrive. The main selenium indicator plants are Astragalus species (including some locoweeds), prince's plume (Stanleya sp.), woody asters (Xylorhiza sp.), and false goldenweed (Oonopsis sp.) Detection in biological fluids Selenium may be measured in blood, plasma, serum, or urine to monitor excessive environmental or occupational exposure, to confirm a diagnosis of poisoning in hospitalized victims, or investigate a suspected case of fatal overdose. Some analytical techniques are capable of distinguishing organic from inorganic forms of the element. Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body prior to elimination in the urine. Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations. Toxicity Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. This 400 µg Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992. The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to avoid creating an imbalance of nutrients in the diet and to accord with data