THYROID RESPONSE TO PARTIAL THYROIDECTOMY, THYROXINE AND 2,4 DINITROPHENOL IN RATS WITH HYPOTHALAMIC LESIONS12

Abstract

Neural regulation of thyrotrophic function of the anterior pituitary has been investigated in the rat. Using in vivo thyroid I131 release as a measure of thyroid activity, the effects of thyroxine, partial thyroidectomy and 2,4 dinitrophenol (DNP) have been studied in normal rats and in rats with hypothalamic lesions that had produced a reduction in baseline thyroid function. L-thyroxine injections, 15 [mu]g. per day, inhibited thyroid activity in 22 operated and 8 control rats to an equal degree. Partial thyroidectomy was followed by functional activation of the remnant in 14 out of 15 rats with lesions and in all of 12 control animals; thyroid regeneration occurred in all control rats, but was blocked in 7 of 15 rats with lesions. DNP produced thyroid inhibition in all of 46 control animals. In 56 rats with lesions DNP was significantly less effective in causing thyroid inhibition than in normal rats and in 5 animals thyroid activity was increased after DNP administration. These observations are interpreted to indicate that the anterior pituitary gland is itself responsive both to increased and to decreased blood thyroid hormone concentration, but that the "setting" at which the pituitary reacts is governed by a nervous system mechanism which involves the hypothalamus. It is concluded that DNP acts on more than one site in inhibiting pituitary thyrotropin release and that there is an important DNP-sensitive neural component in this system.