Effect of thyrotopin and the iodine content of the thyroid on the triiodothyronine: thyroxine ration of newly synthesized iodothyronines.

Abstract

Massive doses of TSH (up to 13.5 U twice daily) given to rats fed a high iodine diet only slightly increased the 131I-labeled T3/T4 ratio (T3*/T4*)2 of newly iodinated thyroglobulin. Moderate doses of TSH (2 U daily) restored a proportionately higher rate of T3* than of T4* synthesis when administered to chronically hypophysectomized, iodine-depleted rats (T3*/T4*>1). Large doses of TSH (10 U daily) given to chronically hypophysectomized rats with a high thyroid iodine content only slightly inincreased T3*/T4* to 0.3 (us. 0.2 in intact controls). Doses of TSH which produced a high T3*/T4* in iodine-depleted, hypophysectomized rats did not do so in rats fed an iodine-deficient diet for 5 days to deplete the extrathyroidal iodide pool while maintaining a relatively high intrathyroidal iodine content. Pretreatment of these rats with 5 mg of perchlorate 30 min before radiodine administration to further reduce the quantity of iodide entering the thyroid did not increase the effectiveness of TSH on T3*/T4*. It ...