STUDIES ON THE METABOLISM OF THYROTROPIN1

Abstract

RAPID disappearance from the plasma of large exogenous doses of heterologous thyrotropin has been reported in man (1), rabbit (2), and rat (3, 4). Less than 5% of the administered dose of thyrotropin remained in rat plasma one hour after intravenous injection (3). Both thyroid (5–7) and liver (8) have been implicated, under in vitro conditions, as the site of inactivation of thyrotropin, although there is in vivo evidence that inactivation may be related primarily to general body metabolism rather than to the function of the thyroid gland per se (4). It has been reported that thyrotropin accumulates in liver (9) or kidney (10) within a few minutes after intravenous injection, and persists in the thyroid gland as long as 24 hours after injection (9). However, it must be pointed out that in one of these studies (9), S³⁵-labeled thyrotropin was detected in the organs by its radioactivity rather than by biological activity.