Effect of 6-n-Propylthiouracil Upon Deiodination of Diiodotyrosine by Normal and Hypophysectomized Rat Thyroids and Liver

The effect of 6-n-propylthiouracil administration on the L-DIT deiodination of thyroid and liver has been studied in normal and hypophysectomized rats. White male rats were divided into 2 groups. The first group was fed for 6 weeks Purina chow containing 0.3 % 6-n-propylthiouracil. The control group was fed Purina chow. At 10 days after starting the treatment and rising linearly during the subsequent 30-day period, the 6-n-propylthiouracil- blocked thyroids of the normal rats showed a highly significant increase of the deiodinating activity when comparable amounts of tissue were homogenized and incubated in the presence of L-DIT-131I. No difference in deiodination was observed when studying the glands of the hypophysectomized rats given 6-n-propylthiouracil or on a regular diet. Both these groups showed a considerably lower deiodinating activity than the normal controls. TPNH enrichment of such homogenates enhanced deiodination in a significant lesser degree than in the normal rats. In contrast with the thyroid tissue, the deiodinating activity of the liver homogenates from rats normal, hypophysectomized or treated with 6-n-propylthiouracil was unchanged. The results suggest that: (a) the highly increased deiodinating activity of the blocked glands of treated normal rats might be due to the results of a prolonged stimulation with endogenous TSH; (b) hypophysectomy prevents the 6-n-propylthiouracil effects on L-DIT deiodination and induces a fall in the deiodinating enzyme concentration or activity in the thyroid tissue. (Endocrinology75: 747, 1964)