IodothyrinI`o*do*thy"rin\, n. [Iodo- + thyro- + -in.] (Physiol. Chem.) A peculiar substance obtained from the thyroid gland, containing from nine to ten per cent of iodine. Note: It is a very stable compound, and is believed to be active principle in thyroid extracts and in the internal secretion of the thyroid gland. It was originally called thyroiodin.