The effect of thiouracil on the metabolism of l-thyroxine

Abstract

The rat conjugates thyroxine chiefly with glucuronic acid and to a very small extent also with sulfuric acid and with an unidentified substance. After the administration of radioactive thyroxine the rat excretes larger amounts of thyroxine glucuronide and smaller amounts of 3,3'',5''-triiodo-thyronine glucuronide in the bile than the dog does. Previous studies showing that deiodination of thyroxine from the 3''- or 5''-position is decreased in rats after receiving thiouracil have been confirmed. Deiodination from the 3- or 5-position of thyroxine is increased in rats given a diet containing 0.1% of thiouracil for 28 days or longer. This may account in part for the marked reduction in calorigenic activity of thyroxine administered to thiouracil-treated rats.