Concomitant Decrease of the Effects of Thyroxin on TRH-Induced TSH Release, and of the Pituitary Content of Triiodothyronine in Animals on Propylthiouracil112

To assess the role of T3 derived from T4 in the regulation of TSH release by T4, we have studied the effects of propyl-2-thiouracil (PTU) on a) the activity of T4 as inhibitor of TRH-induced TSH release and b) on the pituitary concentration of T3 derived from T4. Rats were thyroidectomized and injected ip once daily with 1.75 μg/100 g body wt T4 (or 125I-labeled T4) for 2 weeks. Half of the animals received saline (C rats), while the remainder received 2 mg of PTU ip just after the last two T4 doses. 7–8 hr later blood samples were obtained at 0, 5 and 10 min after 100 ng TRH iv. When labeled T4 was used, rats were perfused at the end of the TRH-stimulation test and the pituitaries were collected for chromatographic separation of the labeled iodine-containing compounds derived from T4. The TRH-induced increase in plasma TSH was greater for PTU than for C rats. The amount of T3 derived from T4 in the pituitaries of the PTU-treated rats was smaller than that found for the C animals, and the amount of T4 was...