Abstract
Estimates were made of I131 incorporated into monoiodotyrosine, di-iodotyrosine, thyroxine and tri-iodothyronine of the thyroid gland from 30 minutes to 48 hours after injection of I131. At early time intervals the estimated specific activity of monoiodotyrosine exceeds that of diiodo-tyrosine. The total radioactivity in thyroxine exceeded that in tri-iodothyronine at all time intervals studied, but the estimated specific radioactivity of the latter compound was higher up to 8 hours after injection of I131. The mechanism of tri-iodothyronine biosynthesis is discussed.