234 /. J. Christian 



injection into the anterior pituitary is followed by a more immediate re- 

 sponse (Yamada and Greer, 1959; Yamada, 1959). 



The thyroid was blocked in these studies (D'Angelo and Traum, 1958) by 

 hypothalamic lesions or by treatment with propylthiouracil. Thyroid func- 

 tion was evaluated by the rate of turnover of radioiodine, bioassay of the 

 pituitaries and plasma for their content of TSH and thyroid hormone, and 

 was finally correlated with the results of detailed histologic studies of the 

 glands of experimental and bioassay animals. The results appear to be 

 definitive. These workers were further able to demonstrate the independ- 

 ence of ovarian, adrenocortical, and thyroid-regulating parts of the hypo- 

 thalamus. How the adrenal corticoids and adrenocorticotropin fit into the 

 regulation of thyroid function is unknown at present. 



4. Thyroid Responses to External Stimuli 



Probably the best-known action of the tl\yroid is its ability to increase 

 the secretion of thyroid hormone in response to cold and to decrease its re- 

 lease in response to heat (Ring, 1942; Brown-Grant et at., 1954a; Stevens 

 et al., 1955; Hellman and Collins, 1957). The gland also becomes hyper- 

 plastic with prolonged exposure to cold (Money, 1955). Upon exposure to 

 low temperatures, there is an increased release of thyroid hormone from 

 the thyroid gland (Brown-Grant et al., 1954a; Woods and Carlson, 1956), 

 apparently in response to increased secretion of TSH by the pituitary 

 (Stevens et al., 1955) . However, Ring (1942) found that the thyroid per se 

 can account for only a small part of the increase in heat production required 

 to meet the demands of mice exposed to 2-4° C, and that epinephrine and 

 thyroid hormone are required for a maximum continued metabolic response 

 to cold. He therefore concluded that the principle action of thyroid in re- 

 sponse to cold exposure was to sensitize the animal to the calorigenic action 

 of epinephrine. Swanson (1956, 1957) essentially confirmed and extended 

 Ring's findings and conclusions, as we have seen above. Thyroidectomy 

 abolishes the response to cold and deprives the animal of the ability to 

 survive exposure to cold (Swanson, 1957). 



Exposure to cold increases pituitary-adrenocortical as well as thyroidal 

 activity, contrary to the more usual reciprocal relationship between ACTH 

 and TSH secretion by the pituitary described below. Increased thyroidal 

 activity apparently is the more vital of these two adaptive responses to 

 cold. Thyroxine permits survival of hypothyroid rats subjected to cold, 

 whereas adrenocorticotropin and adrenocortical extract do not (Freedman 

 and Gordon, 1955). Similarly, minute amounts of thyroxine permitted 

 100% survival of rats treated with thiouracil upon exposure to cold, 

 whereas adrenocortical extracts failed to increase survival (Ershoff, 1948). 



