EFFECT OF GRADED DOSES OF PROPYLTHIOURACIL ON THYROIDAL HORMONE SYNTHESIS IN THE HYPOPHYSECTOMIZED AND IN THE THYROXINE-TREATED RAT

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Abstract
The inhibition of thyroidal hormone synthesis by propylthiouracil (PTU) was studied in intact, hypophysectomized and thyroxine-treated rats. Animals were fed a low iodine diet for 7 days before hypophy-sectomy or before thyroxine injection was started and continued on this diet for one more week up to autopsy. Graded doses of PTU were administered once daily for the last 2 days before autopsy. Radioiodine was given intraperitoneally immediately after the last dose of PTU and thyroids were removed 24 hours later. In intact animals fed a low iodine diet for 2 weeks, the formation of T3 + T4 was the most easily, the formation of DIT next and the formation of MIT the least easily inhibited by PTU. In contrast, the formation of MIT and DIT in hypophysectomized and in thyroxine-treated animals given graded doses of PTU was apparently inhibited to an equal extent No iodothyronines were formed in the hypophysectomized rats and very little was formed in the thyroxine-treated rats which did not receive PTU. Approximately 10 times as much PTU was required to depress MIT and DIT formation to an equivalent extent in the thyroxine-treated as in the hypophysectomized animals.