2. Endocrines and Populations 235 



Heat, on the other hand, diminishes thyroid activity (Hellman and Collins, 

 1957) . Rats and mice have been shown to have reduced thyroid activity in 

 the warm summer months (Hurst and Turner, 1947) . Puntriano and Meites 

 (1951) suggested that the seasonal changes in thyroid activity might be 

 due to changes in day length rather than to seasonal changes in tempera- 

 ture, on the basis of their findings that prolonged exposure to light inhibits 

 thyroid activity in mice and exposure to darkness increases thyroid activity. 

 However, these results could not be confirmed in rabbits (Brown-Grant 

 et al., 1954b) . The rate of release of radio iodine by the thyroids of rabbits 

 was unaffected by prolonged exposure either to darkness or to light. There- 

 fore the validity of the hypothesis that day length affects thyroid activity 

 is dubious. 



In contrast to the thyroidal-adrenocortical relationship in response to 

 exposure to cold, there is abundant evidence indicating that the activity of 

 the thyroids is inhibited in response to alarming stimuli which evoke an 

 increased pituitary-adrenocortical activity. In fact it has come to be 

 generally accepted that there is a reciprocal relationship between ACTH 

 and TSH secretion in response to "stress" (Harris, 1955a). Anoxia, neph- 

 rectomy, vitamin deficiencies, tourniquet shock, fasting, injected typhoid 

 vaccine, swimming in cold water (15° C), injections of formalin, spinal 

 cordotomy, electroshock, and gastrointestinal and peripheral trauma all 

 inhibit the activity of the thyroid glands of rats (Williams et al, 1949; 

 Paschkis et al, 1950; Bogoroch and Timiras, 1951; Hamolsky et al, 1951; 

 Van Middlesworth and Berry, 1951; Badrick et al, 1954, 1955). Similar 

 results were obtained with rabbits, using the release of radioiodine as the 

 basis of measurement (Brown-Grant et al, 1954b) . Hemorrhage, anesthesia, 

 laparotomy, intraperitoneal injections of turpentine, draining subcutaneous 

 abscesses, or emotional stress in the form of restraint, subcutaneous faradic 

 stimulation, or sudden changes from light to dark or vice versa were all 

 found to inhibit thyroid activity in rabbits for 1-2 days. After this time 

 the rabbits evidently became used to the procedures and were no longer 

 alarmed by them, as thyroid function gradually returned to normal with 

 continued exposure to these emotional stimuli. In some instances of these 

 experiments the release of radioiodine from the thyroid was totally inhi- 

 bited (Brown-Grant et al, 1954b). Emotional stimuli also were followed 

 by a time lag of somewhat less than 3 hours between their first application 

 and the first thyroid response. These experiments demonstrate that emo- 

 tional factors, visual stimuli, and the central nervous system can affect 

 thyroid function, presumably via the hypothalamus, just as they can in- 

 crease the secretion of ACTH or adrenal medullary hormones (Brown- 

 Grant et al, 1954b). Kracht (1954) described a "true thyrotoxicosis" in 

 wild rabbits stimulated by fear in response to being chased by ferrets and 



