Chronic regulation of rat thyroid prostaglandin synthetase activity by endogenous thyrotropin

Prostaglandin (PG) synthetase activity was measured in the rat thyroid after hypophysectomy, T4, and methylthiouracil treatment in order to explore the possibility that PG synthetase is subject to chronic control by endogenous TSH. T4 treatment (5 mg/liter drinking water) lowered PG synthetase activity, as measured in whole homogenates, by 22% and 31% after 2 and 4 days, respectively. Similarly, PG synthetase activity was reduced by 16% and 42% 2 and 4 days after hypophysectomy. Methylthiouracil treatment (100 mg/liter drinking water) effected 51% and 78% increases in synthetase activity by days 2 and 4, respectively. Additionally, differential effects with regards to PGE2 to PGF 2 alpha ratios were observed. The limited prostaglandin-catabolizing capacity of the rat thyroid warrants the conclusion that the observed TSH effects are expressed at the level of the PG synthetase complex.