The Collaborative International Dictionary of English v.0.48:

Iodothyrin \I`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.
         [Webster 1913 Suppl.]

   Note: It is probably an impure extract of thyroid gland,
         containing thyroxine. See Merck Index 11th Ed. #9345,
         the entry for thyroidin.
         [PJC]