Effect of Thyrotropin Injection on Pyridine Nucleotide Content of Thyroid in Normal and Hypophysectomized Rat

Abstract

Hypophysectomized rats were given thyrotropin (TSH), growth hormone (GH) or TSH+GH for 21 days and at various intervals during these regimens the deiodination of I131-labeled diiodotyrosine (L-DIT) was measured in homogenates of thyroid and liver. The thyroidal deiodination of L-DIT did not differ from controls 24 hr after one injection of 2 U USP TSH, but was greatly increased after 7 daily injections. The response of the deiodinating activity as well as of serum PBI and thyroid weight in the TSH/treated group suggests development of resistance to TSH beginning after the 10th day of treatment. Significant titers of anti-TSH antibodies were detected in sera of rats treated for 21 days with TSH. The hepatic deiodination was not affected by TSH administration. GH injection (200 [mu]g/day) for more than 7 days enhanced considerably the thyroidal and, after 21 days, the hepatic deiodinating activity. GH+TSH administration induced greater responses in deiodination, serum protein-bound iodine and thyroid weight than those seen with either alone. These effects suggest an enhancement of the GH activity by the higher levels of circulating thyroid hormones observed in this group, since they were found at a time (14 and 21 days of treatment) when those of TSH on the thyroid were no longer evident, and since TSH has no influence on the liver. The results indicate that both GH and TSH influence the thyroidal deiodinating activity, and that chronic administration of GH can stimulate the hepatic deiodination of L-DIT.