Plays a crucial role in the metabolism of thyroid hormones (TH) and has specific roles in TH activation and inactivation by deiodination (PubMed:12586771, PubMed:32718224, PubMed:18821722). Catalyzes the deiodination of L-thyroxine (T4) to 3,5,3'- triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3) to 3,3'- diiodothyronine (3,3'-T2) and 3',5'-diiodothyronine (3',5'-T2) to 3'- monoiodothyronine (3'-T1) via outer-ring deiodination (ORD) (PubMed:32718224, PubMed:18821722, PubMed:12586771, PubMed:18339710). Catalyzes the deiodination of T4 to 3,3',5'-triiodothyronine (rT3) via inner-ring deiodination (IRD) (PubMed:32718224). Catalyzes the deiodination of T3 to 3,3'-T2, 3,5-diiodothyronine (3,5-T2) to 3- monoiodothyronine (3-T1) and 3,3'-T2 to 3-T1 via IRD (By similarity). Catalyzes the phenolic ring deiodinations of 3,3',5'- triiodothyronamine and 3',5'-diiodothyronamine (PubMed:18339710). Catalyzes the phenolic ring deiodination of 3,3'-diiodothyronamine and tyrosyl ring deiodinations of 3,5,3'-triiodothyronamine and 3,5- diiodothyronamine (By similarity). Catalyzes the deiodination of L- thyroxine sulfate and 3,3',5-triiodo-L-thyronine sulfate via IRD and of 3,3',5'-triiodo-L-thyronine sulfate via ORD (By similarity). {ECO:0000250|UniProtKB:P24389, ECO:0000250|UniProtKB:Q2QEI3, ECO:0000250|UniProtKB:Q61153, ECO:0000269|PubMed:12586771, ECO:0000269|PubMed:18339710, ECO:0000269|PubMed:18821722, ECO:0000269|PubMed:32718224}. This is the function of ENSG00000211452 (DIO1, iodothyronine deiodinase 1).