Evidence for a thyroid-inhibiting factor of adenohypophysial origin

Abstract

Thyroids of normal or thyroxine ( T4)-pretreated rats were incubated in vitro in a serum medium containing I131. It was found that the addition of either a whole rat adenohypophysis, a crude rat anterior pituitary extract, or of commercial bovine thyrotrophin (TSH) to the medium caused a slight stimulation of I131 release and no apparent stimulation but rather an immediate and considerable inhibition of thyroid-I131 uptake. Preincubation of rat thyroid with crude anterior pituitary extract resulted in a prolonged inhibitory effect on the thyroid-I131 uptake. In vivo studies showed that shortly after TSH administration a similar inhibition of uptake occurred in normal rats, although allowing 24 hr for TSH stimulation brought about no change in iodine uptake in thyroids of normal and a marked increase in uptake by thyroids of T4-pretreated rats. The inhibition of thyroid-I131 uptake was assumed to have been caused either by TSH itself, or by a thyroid-inhibiting factor of adenohypophysial origin present in commercial TSH preparations as a contaminant.