2. Endocrines and Populations 229 



be able to maintain a constant internal environment in the face of stimuli 

 from and changes in the external environment. The thyroid is intimately- 

 associated with a number of adaptive mechanisms and has important 

 interractions with the adrenal cortex, as well as with the adrenal medulla 

 and its hormones. This is not the place for a detailed account of thyroid 

 physiology, but a brief review of the functions and actions of the thyroid 

 and its hormones will be given, largely derived from standard accounts 

 and reviews. Emphasis will be placed on the role played in physiologic 

 adaptation to environmental changes, especially in response to adverse 

 stimuli. 



It has long been known that thyroid hormone is essential for normal 

 growth and development, as well as for the normal metabolism of most 

 tissues. Furthermore it has a vital role in permitting a mammal to adapt 

 to changes in the temperature of the external envirormient, especially in 

 adaptation to cold, by acting synergistically with the calorigenic action of 

 epinephrine, as we have seen in the preceding section, as well as by playing 

 an important physiological role of its own (Swanson, 1957) . The evidence 

 that will be discussed subsequently shows that the thyroid also is involved 

 intimately in the physiological responses to alarming stimuli. Therefore 

 the thyroid, like the adrenal gland, assumes particular importance in a 

 discussion of adaptive mechanisms. 



2. The Thyroid Hormones and Their Actions 



The thyroid hormones, tetraiodothyronine (thyroxine) and Z-3,5,3'- 

 triiodothyronine, have an overall action of increasing heat production by 

 increasing the oxidative processes of many tissues and therefore their 

 oxygen consumption (SoUman, 1957). This metabolic effect is in part 

 brought about by the augmentation or facilitation of the calorigenic action 

 of epinephrine, and it has been suggested that thyroxine and epinephrine 

 influence consecutive rate-limiting reactions in the metabolic cycle, thyrox- 

 ine acting at a later stage than epinephrine (Swanson, 1956) . The meta- 

 bolic effect of thyroxine, however, is not exerted equally on all tissues. The 

 rate of oxidation by brain tissue, for example, is not influenced at all (Tata 

 et al., 1957), whereas the metabolism of the liver is increased more than of 

 the body as a whole (Barker and Schwartz, 1953). However, the electro- 

 encephalogram excitability of the brain, electrolyte distribution, and circu- 

 lation of the brain are profoundly affected by thyroid hormones (Tata 

 et al., 1957). Increased thyroid hormone first affects carbohydrate, then 

 fat, and finally protein metabolism (Sollman, 1957) . Conversely, a defi- 

 ciency in thyroid hormone reduces the oxidative activity of tissues in 

 general. It has been suggested that all the actions of the thyroid hormones 



