Idothyroxine, also known as 3,5-diiodothyronine (T2), is a thyroid hormone derivative that plays a role in regulating metabolism and energy production in the body. It is less well-known than its more prominent counterparts, thyroxine (T4) and triiodothyronine (T3). Research has suggested that idothyroxine may have effects on metabolic processes and could potentially have therapeutic implications, though its role and efficacy are still being studied.