REGULATION OF THYROTROPIN SECRETION BY EXOGENOUS THYROXINE AND TRIIODOTHYRONINE

The inhibition of thyrotrophin secretion by oral doses of 3,5,3'-triiodo-thyronine and thyroxine was studied in subjects with suppressed thyroidal secretion to test the hypothesis that thyroxine has intrinsic metabolic activity in the control of TSH secretion. Five previously untreated female patients with hyperthyroid Graves' disease were given 40 mg carbimazole daily throughout the study. After six weeks of treatment, thyroid hormone replacement was started. In phase 1, they received 20 μg triiodothyronine/day for six weeks, which was then increased to 30 μg/ day, 40 μg/day and 50 μg/day at weekly intervals. In phase 2, the patients received 20 μg triiodothyronine/day for five weeks. Thyroxine supplements were then added to the regimen commencing with 50 μg/day for one week, then 100 μg/day followed by 150 μg/day, each for one week. Total and free triiodothyronine and thyroxine, total 3,3',5'-triiodothyronine (reverse triiodothyronine) and basal TSH were measured by radioimmunoassay in serum samples taken before each change in thyroid hormone supplement. In both phases of the study, a stepwise increase in triiodothyronine concentration was associated with a fall in thyrotrophin concentration. The inhibition of thyrotrophin secretion in phase 2 in relation to serum triiodothyronine concentration was parallel to that observed in phase 1 despite a progressive increase in thyroxine to levels in the low normal range. It has not been possible to demonstrate that thyroxine, at the concentration studied, plays a role in the regulation of TSH secretion independently of its conversion to triiodothyronine. © 1979.