title: Feedback regulation of thyrotropin by thyroxine under physiological conditions.
abstract: 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.
mesh_terms: D000818:Animals; D005246:Feedback; D005260:Female; D007037:Hypothyroidism / Q000097:blood* / Q000188:drug therapy; D007263:Infusions, Parenteral; D010902:Pituitary Gland / Q000378:metabolism; D011863:Radioimmunoassay; D051381:Rats; D013961:Thyroid Gland / Q000502:physiology; D013972:Thyrotropin / Q000378:metabolism*; D013974:Thyroxine / Q000008:administration & dosage / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine