Deiodinase, Iodothyronine, Type I (DIO1)

DIO1 is a thiol-requiring propylthiouracil-sensitive oxidoreductase. It activates thyroid hormone by converting the prohormone thyroxine (T4) by outer ring deiodination (ORD) to bioactive 3,3',5-triiodothyronine (T3). It also degrades both hormones by inner ring deiodination (IRD). Alternative splicing results in multiple transcript variants encoding different isoforms. Some, but not all, isoforms contain a selenocysteine (Sec) residue encoded by the UGA codon, which normally signals translation termination. The 3' UTR of Sec-containing genes have a common stem-loop structure, the sec insertion sequence (SECIS), which is necessary for the recognition of UGA as a Sec codon rather than as a stop signal. Additional transcript variants have been described but are not supported by experimental evidence.