Several parameters of the effects of thyroid hormone were examined in hypothyroid thyroidectomized (Tx) rats treated with T3 sulfate (T3S) or 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 significantly after treatment with both doses of T3S and T3. The mean serum GH level was markedly reduced in Tx rats, and it improved significantly to similar levels after treatment with the high dose of T3S and the low dose of T3. Type I monodeiodinase (MD) activity was markedly reduced in liver and kidney tissues of Tx rats. It increased significantly in Tx rats treated with the high dose of T3S; the latter values were similar to those observed in Tx rats treated with the low dose of T3. Hepatic and renal type I MD activities increased to supranormal levels in Tx rats treated with the high dose of T3. Cardiac outer ring (5') monodeiodination of 3',5'-diiodothyronine to 3'-monoiodothyronine was 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 restored to normal in Tx rats treated with the high dose of T3S and both doses of T3. Serum TSH, markedly elevated in Tx rats, was appreciably reduced only in rats treated with the high dose of T3. In another study, significant suppression of serum TSH was observed when Tx rats were treated with T3S (11.5 nmol/day) or T3 (2.3 nmol/day) for 3 days. We conclude that administration of T3S to hypothyroid rats produces thyromimetic effects, with a potency approximately one fifth that of T3.