2. Endocrines and Populations 233 



thyroid activity, probably by inhibiting the release of TSH from the pitui- 

 tary. 



Epinephrine, in addition to its synergism with thyroxine with respect to 

 calorigenesis, appears to have more direct effects on thyroid activity, al- 

 though the reports are conflicting. Badrick and his co-workers (1954, 1955) 

 reported that epinephrine inhibited the uptake of radioiodine by the thy- 

 roids of intact and hypophysectomized rats, and similar results were re- 

 ported for intact rats by Money et al. (1950). Brown-Grant et al. (1954b) 

 found that epinephrine decreased the release of radioiodine by the thyroids 

 of intact rabbits. On the other hand, epinephrine has been reported to in- 

 crease the release of radioiodine by the thyroid (Williams et al., 1949) and 

 to increase the content of radioiodine in the thyroid (Botkin and Tew, 

 1952), although the latter could result from an inhibition of its release. 

 More recently Ackerman and Arons (1958) have reported that epinephrine 

 increases the release of radioiodine from the thyroids of intact and hypo- 

 physectomized dogs. These divergent results are difficult to reconcile, al- 

 though the fact that epinephrine acted in hypophysectomized as well as in 

 intact rats and dogs (Badrick et al, 1955; Ackerman and Arons, 1958) 

 suggests that the effects were directly on the thyroid gland. It is possible 

 that these varying results may reflect the effects of epinephrine on the 

 thyroidal vasculature in different animals and different circumstances, as 

 was suggested by Badrick et al. (1955) . 



From the foregoing account it should be clear that the regulation of 

 thyroid function is complex and involves a number of factors. However, 

 there seems to be little doubt that the hypothalamus has a relatively basic 

 role in regulating the secretion of thyrotropin from the pituitary, and evi- 

 dently is capable of inhibiting its release (Brown-Grant et al., 1954b; Reich- 

 lin, 1957a; Harris and Woods, 1958; D'Angelo and Traum, 1958) . D'Angelo 

 and Traum (1958), in an extensive series of experiments on hypothalamic- 

 hypophyseal-thyroidal relationships in rats, concluded that the hypothala- 

 mus functions as a modulator, modifying the production and release of 

 thyrotropin from the adenohypophysis not only under conditions of in- 

 creased demand, but also in meeting day-to-day needs. Normal blood levels 

 of TSH cannot be maintained in the absence of stimulation by the hypo- 

 thalamus even with decreased levels of thyroid hormone. However, the 

 adenohypophysis keeps its capacity to produce and release TSH, but regu- 

 lated by the level of circulating thyroid hormone or by peripheral meta- 

 bolic actions thereof (cf. above). The action of thyroid hormone may be 

 directly on the pituitary (D'Angelo and Traum, (1958). The injection of 

 minute amounts of thyroxine into the hypothalamus is followed by a rela- 

 tively long latent period before the release of TSH is inhibited, whereas 



