Rapid adaptation of pituitary responsiveness to TRH in the post-surgical state. The role of free T3.

In eight clinically and biochemically euthyroid patients undergoing routine major non-thyroidal surgery preoperative and daily postoperative serum concentrations of total and free thyroid hormones were measured. Thyrotrophin-releasing hormone (TRH) tests were performed preoperatively and on the first 3 postoperative days. There was a significant fall in mean serum total and free triiodothyronine (T3) concentrations on the postoperative days and mean reverse T3 concentrations rose reciprocally. There was no significant change in mean basal thyroid-stimulating hormone (TSH) values, but there was a significant increase in the mean TSH-response to TRH on the first postoperative day. The mean TSH response than declined sequentially until day 3 while mean free T3 concentrations remained significantly depressed. Mean serum free thyroxine(T4)concentrations remained normal during the study. Intrapituitary conversion of T4 to T3 or other down regulatory mechanisms could explain this rapid adaptation of the pituitary axis.