The function of DIO3 (iodothyronine deiodinase 3, Ensembl gene identifier ENSG00000197406) is as follows. Plays a crucial role in the metabolism of thyroid hormones (TH) and has specific roles in TH activation and inactivation by deiodination (PubMed:7593630, PubMed:12586771, PubMed:12746313, PubMed:18821722). Catalyzes the deiodination of L-thyroxine (T4) to 3,3',5'-triiodothyronine (rT3), 3,5,3'-triiodothyronine (T3) to 3,3'- diiodothyronine (3,3'-T2), 3,5-diiodothyronine (3,5-T2) to 3- monoiodothyronine (3-T1), rT3 to 3',5'-diiodothyronine (3',5'-T2) and 3,3'-T2 to 3'-monoiodothyronine (3'-T1) via inner-ring deiodination (IRD) (PubMed:7593630, PubMed:12586771, PubMed:12746313, PubMed:18821722, PubMed:18339710). Catalyzes the deiodination of 3-T1 to L-thyronine (T0) via outer-ring deiodination (ORD) (PubMed:18821722). Catalyzes the tyrosyl ring deiodinations of 3,3',5,5'-tetraiodothyronamine, 3,3',5'-triiodothyronamine, 3,5,3'- triiodothyronamine, 3,5-diiodothyronamine, 3,3'-diiodothyronamine and 3-iodothyronamine (PubMed:18339710). {ECO:0000269|PubMed:12586771, ECO:0000269|PubMed:12746313, ECO:0000269|PubMed:18339710, ECO:0000269|PubMed:18821722, ECO:0000269|PubMed:7593630}.