IT HAS generally been held that hormone secretion by the thyroid and thyrotrophin (TSH) output by the anterior pituitary are homeostatically associated in a “feed-back” type relation. Thus, Hoskins (1) has stated: “When the titer of circulating thyroxine rises, the anterior pituitary is selectively inhibited and the discharge of thyrotrophin is thereby decreased. Contrariwise, episodic or persistent thyroxine deficiency, if sufficient in degree, results in augmented thyrotrophin production with resulting tendency for the production of more thyroid hormone.” Most available data on thyroid activity and pituitary TSH output have been consistent with this view; however, evidence from a number of recent studies has indicated that the actual relationship is perhaps considerably more complicated.