Feedback regulation of thyrotropin by thyroxine under physiological conditions

We have studied the contribution of thyroxine (T4) itself to the feedback regulation of thyrotropin (TSH) secretion. Thyroidectomized rats received T4 replacement by one of two methods: 1) 5 or 10 micrograms T4 . 100 g-1 . day-1 in drinking water or 2) continuous subcutaneous infusion of 0.5, 1.0, or 2.0 micrograms T4 . 100 g-1 . day-1. Replacement of 5 micrograms T4 . 100 g-1 . day-1 resulted in elevated plasma T4 and TSH while triiodothyronine (T3) was decreased. Replacement of 10 micrograms T4 . 100 g-1 . day-1 resulted in greater elevation of plasma T4, a transient decrease in T3, and no change in plasma TSH. Continuous replacement of T4 resulted in a dose-dependent elevation of plasma T4, little plasma T3 generation, and inhibition of the postthyroidectomy rise in plasma TSH. The pituitary responsiveness to thyrotropin-releasing hormone (TRH) was increased in groups receiving 1.0 or 2.0 micrograms T4 . 100 g-1 . day-1. It was concluded that 1) the plasma T4 exerts a negative feedback on basal TSH secretion in addition to that due to plasma T3 and 2) small amounts of T4 replacement enhance the TSH response to exogenous TRH in short-term hypothyroid rats.