Effects of Thyroid Hormones on Ring A Reduction of Cortisone by Liver

Abstract

Triiodothyronine increases total hepatic capacity for in vitro reduction of ring A of cortisone by 38% in both male and female rats. Thyro-parathyroidectomy decreases it 56% in males and 48% in females. These alterations in thyroid state influence hepatic reduction of corticosteroids in several ways: a) the total amount of Δ⁴-steroid hydrogenases in the liver may be altered, both by changes in liver size and by changes in amount of enzyme per gram of liver; and, b) the activity per gram of liver can also be altered through variations in coenzyme (TPNH) availability. These effects of thyroid hormones provide an enzymatic basis for the alterations in biological half-life of adrenal steroids observed in hyper- and hypothyroidism. A very close correlation ( r = 0.97) between total hepatic capacity to inactivate cortisone and the size of the adrenal glands was found. It is suggested that the rate of ring A reduction of corticosteroids by liver determines the rate of ACTH secretion in unstressed animals.