2. Endocrines and Populations 239 



hormone decreased the weights of the reproductive organs. However, thy- 

 roidectomy was followed by an increase in the relative weights of the testes 

 and seminal vesicles of rats (Hess, 1953). Hess (1953) concluded that 

 thyroidectomy appears to sensitize the gonads and secondary sex organs 

 to the actions of gonadtropins. Hyperthyroidism is associated with ab- 

 normal reproductive function in female rats (Denison and Zarrow, 1955). 

 Prolonged exposure to 2° C. resulted in a marked increase in the length of 

 the estrous cycle: proestrus and estrus were markedly prolonged, while 

 metestrus and diestrus were shortened. Treatment with 50 fxg. of thyroxine 

 corrected these changes. On the other hand, brief (3 days) treatment of 

 female rats with thyroid hormone just prior to mating apparently resulted 

 in an increase in subsecjuent litter size, but the same amount of thyroid 

 hormone given during hot weather or an increased amount of thyroid 

 hormone adversely affected reproduction (Kraatz, 1939) . These results 

 seem to indicate that there is an optimal level of thyroid function for proper 

 functioning of the reproductive system, and that any appreciable deviation 

 from this level in either direction leads to diminished reproductive function. 



The role of the thyroid in reproduction has been investigated by using 

 radiothyroidectomized mice (Bruce and Sloviter, 1957). Radiothyroidec- 

 tomy apparently had no effect on male fertility, female fertility, or litter 

 frequency, but estrus and gestation were prolonged. Litter size possibly 

 was reduced owing to increased resorption of embryos, but the matter 

 requires further investigation. In contrast to these results, Chu (1944) 

 found that surgical thyroidectomy in rabbits during early pregnancy re- 

 sulted in total loss of fetuses due to resorption or abortion. The same opera- 

 tion in later pregnancy resulted in the young being stillborn. Induction of 

 pregnancy in thyroidectomized rabbits was followed by resorption or 

 abortion of the embryos or a prolongation of gestation, the young subse- 

 quently being born dead. Feeding desiccated thyroid tended to prevent 

 the adverse effects of thyroidectomy (Chu, 1944) . Hypothyroidism induced 

 by thiouracil in rats apparently was without effect on fertility in males and 

 females but interferred with gestation in pregnant females, causing resorp- 

 tion in 100% of the animals (Jones et al., 1946). Thiouracil treatment of 

 rats for shorter periods of time, less than 100 days, did not prevent some 

 of the rats from delivering litters, but the young exhibited abnormal 

 growth and development. It is quite evident that pregnancy is adversely 

 affected by severe hypothyroidism with a marked increase in intrauterine 

 mortality. 



In general reproductix'e function is altered by marked increases or de- 

 creases in thyroid activity, and it seems that the depression in thyroid 

 activity in response to alarming stimuli must play a role, along with in- 

 creased adrenocortical activity, in the commonly observed suppression of 



