Selenium also plays a role in the functioning of the thyroid gland.  It participates as a cofactor for the three thyroid hormone deiodinases.  These enzymes activate and then deactivate various thyroid hormones and their metabolites. It may inhibit Hashimotos disease, an auto-immune disease in which the bodys own thyroid cells are attacked by the immune system. A reduction of 21% on TPO antibodies was reported with the dietary intake of 0.2 mg of selenium.