Selenium is one of the most powerful antioxidants known and a component of several powerful antioxidant enzymes. If selenium is in the soil, plants pick it up. If it’s not in the soil, they can’t pick it up. Plants don’t actually need selenium to grow. It helps with their defense against pests, but for the most part, plants will “look” just fine if grown in selenium-deficient soil—especially if doused with pesticides to control for pests. Plants require only 16 nutrients to grow; selenium isn’t one of them. In other words, if selenium isn’t in the soil, the final crop will be selenium-deficient. It will not be found in the plant, either alone or as part of any other molecules.
Selenium soars in importance for the very specific reason that our world is choking on a rising tide of mercury pollution made much worse because of the insane use of mercury in dentistry and medicine. Selenium is a potent immune stimulator—some think the most potent immune stimulator of all. Selenium is an essential component of thyroid metabolism and antioxidant defense, as well as immune function. It may improve activation and proliferation of B-lymphocytes and enhance T-cell function. Thus we should not be surprised to find that those cancer patients with low selenium levels tend to have a wider spread of the disease, more recurrences and die sooner.
A new study in the Journal of Hygiene Research suggests that taking selenium supplements or eating selenium-rich foods may help reduce the damage to neurons and minimize memory loss induced by fluoride, which is added in drinking water in the U.S.
Z. Zhang of Zhejiang Normal University in Jinhua, China and colleagues conducted the study and found supplementation of selenium in a moderate dose can decrease the toxic effect of fluoride. Iodine has this effect as well. Selenium also makes mercury less toxic because it binds so tightly with it. There are many reasons to take selenium and the best one is that it is necessary for glutathione production.
Selenium is necessary for the conversion of T4 to T3. (Incomplete conversion results in high levels of reverse T3, an inactive hormone.) Selenium has also been shown to reduce autoimmunity against the thyroid (i.e. to treat the underlying cause of Hashimoto’s thyroid disease.)
 "16 Elements Necessary for Plants." FloridaGardner.com. 26 May 2009. (Accessed 24 Sept 2012.) http://floridagardener.com/misc/fertifacts.htm