STUDIES ON THE MECHANISM OF HYPOTHALAMIC CONTROL OF THYROTROPIN SECRETION: EFFECT OF THYROXINE INJECTION INTO THE HYPOTHALAMUS OR THE PITUITARY ON THYROID HORMONE RELEASE1

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Abstract
In an attempt to ascertain whether or not thyroxine can change hypothalamic as well as pituitary activity in regulating thyrotropin secretion, microinjections of thyroxine were made directly into a localized area of the hypothalamus or into the anterior pituitary. Determination of thyroid secretion was utilized in evaluating possible changes in thyrotropin secretion. Inhibition of thyrotropin secretion was produced by injection of thyroxine into either the “thyrotropin area” of the hypothalamus or into the anterior pituitary. Injection into the subarachnoid space or into other areas of the hypothalamus was ithout effect. Hypothalamic injections had a latent period of six to nine hours before thyrotropin secretion was inhibited, whereas the inhibition was immediate after pituitary injections. The duration of inhibition was approximately 20 hours in either location. It is suggested that two separate mechanisms might control thyrotropin secretion, a slower one acting through the central nervous system and a direct one in the anterior pituitary itself.