3,5 Diiodo-L-Thyronine is the official medical name for T2 thyroid hormone. Each thyroid hormone has a name based on the number of iodine atoms found bound to it. T4 thyroid hormone is known as thyroxine, T3 thyroid hormone is known as triiodothyronine, and T2 thyroid hormone is known as 3,5 diiodo-l-thyronine. Don't let the name confuse you, though, it's just another way to say T2 thyroid hormone.