THYROMIMETIC EFFECTS OF 3,5,3'-TRIIODOTHYRONINE SULFATE IN HYPOTHYROID RATS

Several parameters of the effects of thyroid hormone were examined in hypothyroid thyroidectomized (Tx) rats treated with T3 sulfate (T3S) o r T3 [0.46 (low dose) or 2.3 (high dose) nmol/day for 10 days, ip]. Tx rats showed a marked degree of growth retardation, which improved sig nificantly after treatment with both doses of T3S and T3. The mean ser um GH level was markedly reduced in Tx rats, and it improved significa ntly to similar levels after treatment with the high dose of T3S and t he low dose of T3. Type I monodeiodinase (MD) activity was markedly re duced in liver and kidney tissues of Tx rats. It increased significant ly in Tx rats treated with the high dose of T3S; the latter values wer e similar to those observed in Tx rats treated with the low dose of T3 . Hepatic and renal type I MD activities increased to supranormal leve ls in Tx rats treated with the high dose of T3. Cardiac outer ring (5' ) monodeiodination of 3',5'-diiodothyronine to 3'-monoiodothyronine wa s also significantly reduced in Tx rats, but it improved significantly only after treatment with the high dose of T3. Type III 5-MD activity was significantly reduced in the cerebral cortex of Tx rats. It was r estored to normal in Tx rats treated with the high does of T3S and bot h doses of T3. Serum TSH, markedly elevated in Tx rats, was appreciabl y reduced only in rats treated with the high dose of T3. In another st udy, significant suppression of serum TSH was observed when Tx rats we re treated with T3S (11.5 nmol/day) or T3 (2.3 nmol/day) for 3 days. W e conclude that administration of T3S to hypothyroid rats produces thy romimetic effects, with a potency is similar to one fifth that of T3.