Deiodinases are selenoproteins. Deiodinases both activate and inactivate thyroid hormones. T3 is the active thyroid hormone. Deficiencies in selenium could lead to decreased activation of T3 by deiodinase I but also decreased inactivation of T3 by deiodinase III whereby individuals would appear to be euthyroid though thyroid hormone homeostasis would be upset. Dysregulation of CTH (cystathionine gamma-lyase), which metabolizes L-seleonomethionine the food form of selenium so selenium can be used to synthesize selenoproteins, could result in dysregulation of DIO1 and DIOIII.