Feedback Regulation of Thyrotropin Secretion by Thyroid Hormones

To the Editor: In his Seminar in Medicine in the January 7 issue, Larsen states that iopanoic acid probably has an in portant effect on intrapituitary conversion of thyroxine (T 4 ) to triiodothyronine (T 3 ), since T 3 replacement in a small group of normal subjects who had received iopanoic acid did not normalize the response of thyroid-stimulating hormone (TSH) to thyrotropin-releasing hormone (TRH). 1 He cites a study of ours 2 as the reference for this statement. However, in our study, T 3 replacement in subjects who had received iopanoic acid did normalize the TSH response to TRH. Subjects . . . No extract is available for articles shorter than 400 words.