Evidence for a thyroid-inhibiting factor of adenohypophysial origin

¹ Cátedra de Fisiopatología, Instituto de Medicina Experimental, Universidad Central de Venezuela, Caracas, Venezuela

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.

Key Words: rat thyroid • I¹³¹ uptake • TSH inhibition of thyroid I¹³¹ uptake • thyroid metabolism • anterior pituitary gland extract and thyroid I¹³¹ uptake

Submitted on February 21, 1963