Effects of thyroxine (T4) and triiodothyronine (T3) on the thyrotropin (TSH) response to TSH-releasing hormone (TRH) in the rat

Wistar rats were treated with 7.8 or 260 nmols T4/100 g BW, 1.5 or 260 nmols T3/100 g BW, or saline as control. Twenty minutes later 1 μg TRH/100 g BW was injected iv. Heparinized blood samples were drawn at times 0 and 30 minutes (10 min post-TRH) for determination of plasma TSH, T4 and T3 by RIA. Other group of rats were administered with 150 μCi of 3',5'- 125I-T4 prepared by iodination of 3,5-diiodothyronine. Thirty minutes later the hypophyses were removed, and chromatographed. Other group of animals were treated with 5 mg of iopanoic acid (IOP)/100 g BW. Thereafter, rats were injected iv with 260 nmols T4 or T3/100 g BW and the TRH-test performed as described above. In the control group there was a 11-fold increase in plasma TSH at 10 minutes post-TRH. In rats treated with 260 nmols T4 the post-TRH increment in plasma TSH was 5+-1-fold (p<0.01); in 7.8 nmols T4 treated group was 7.6+-1.3-fold (p<0.01) and in 260 nmols T3 treated rats 6+-1-fold (p<0.01). Similar results were observed after injecting T4 or T3 with the 260 nmols dose in IOP-treated rats. No significant changes were seen in the response to TRH in rats treated with 1.5 nmols T3 and in plasma T3 after T4 injection. Chromatography showed 4.3% of 125I-T3 in the hypophyses 30 minutes after 125I-T4 administration. The present data indicate that T4 is capable of depressing the release of TSH in response to TRH stimulation in normal rats. (M.E.L.)