The Effect of Hypothyroidism and Thyrotoxicosis on Thyroxine Metabolism in the Rat

Abstract

The effect of alterations in metabolic status on the peripheral metabolism of thyroxine (T4) has been studied in thyroidectomized rats by an isotopic equilibration technique. In hypothyroid animals total T4 clearance rate was decreased as a result of decrease in the activity of both fecal and deiodinative routes of T4 metabolism. T4 distribution space was unchanged or very slightly decreased, and the net thyroid hormone binding affinity of the serum proteins, as judged by in vitro uptake tests, was increased little, if at all. In animals made thyrotoxic by daily administration of 10 M—g T4/ISO g bw daily, total T4 clearance rate was more than doubled, this as a consequence of increases in both fecal and deiodinative clearance. Total T4 distribution space was slightly increased and net thyroid hormone binding affinity of serum slightly decreased. Qualitatively similar results were obtained when animals were made thyrotoxic by administration of 3.3 µg L—triiodothyronine/150 g bw daily. Since the changes in ...