AJP - Endocrinology and Metabolism, Vol 240, Issue 3 308-E313, Copyright © 1981 by American Physiological Society
Feedback regulation of thyrotropin by thyroxine under physiological conditions
J. M. Connors and G. A. Hedge
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.
