DEMONSTRATION OF THYROMIMETIC EFFECTS OF 3,5,3'-TRIIODOTHYRONINE SULFATE (T3S) IN EUTHYROID RATS

We have previously demonstrated that T-3 sulfate (T3S) exhibits thyrom imetic effects in hypothyroid rats and that, on a molar basis, its act ivity approximates 20% that of T-3. Since T3S is avidly deiodinated by 5'-deiodinase, type I (5'-DI) and 5'-DI activity is markedly reduced in hypothyroidism, it seemed possible that T3S is active only in hypot hyroidism and not in the euthyroid state wherein normal tissue 5'-DI a ctivity will rapidly degrade T3S and little T3S will be left for metab olism (desulfation) to biologically active T-3. This Study was underta ken to test this possibility. We studied the effect of T3S (4.6, 14, o r 42 nmol/day for 7 days, ip) and T-3 (1.0, 3.0 or 9.0 nmol/day for 7 days, ip) in groups of male Sprague-Dawley rats (5-6/group); the contr ol group was treated with saline ip. Treatment with both T-3 and T3S c aused a significant (p < 0.05) increase in hepatic and renal 5'-DI. Si milarly, both treatments caused a significant reduction in serum total T-4 and TSH levels. In these effects, T3S was approximately one-fifth as potent as T-3 on a molar basis. Interestingly, changes in body wei ght during treatment with T-3 and T3S suggested that at doses that cau sed a comparable tissue or pituitary effect, T3S treatment permitted a significantly greater weight gain than treatment with T-3. We conclud e that T3S exhibits thyromimetic effects in euthyroid rats in a manner comparable to that in hypothyroid rats. The biological effects of T3S may be due to T-3 generated in tissues by desulfation of T3S.