NUTRITIONAL EFFECTS 



669 



testis. On the other hand, the lack of 

 demonstrable thyroid dysfunction in the 

 adult male does not exclude the possibility 

 of an effect of thyroid hormone on repro- 

 duction. The conversion of thyroxine to tri- 

 iodothyronine may be hindered (Morton, 

 1958) . Thyroxine was found to decrease the 

 number of active cells in the semen and to 

 reduce motility, whereas triiodothyronine 

 increased the number of active spermatozoa 

 (Farris and Colton, 1958; Reed, Browning 

 and O'Donnell, 1958j. Small dosages of thy- 

 roxine stimulated spermatogenesis in the 

 mouse, rabbit, and ram (Maqsood, 1952) 

 and were beneficial in normal guinea pigs 

 and rats (Richter and Winter, 1947; Young, 

 Rayner, Peterson and Brown, 1952). 



In many species reproduction occurs de- 

 spite hypothyroidism, but fecundity may 

 be subnormal (Peterson, Webster, Rayner 

 and Young, 1952). Feeding an antithyroid 

 drug, thiouracil, to female rats may or may 

 not prevent pregnancy, but it reduces the 

 number of young per litter. Thiouracil feed- 

 ing continued through lactation will de- 

 crease litter size (Leathem, 1959b). Hypo- 

 thyroid guinea pigs gave birth to some live 

 young, but the percentage approached nor- 

 mal only when thyroxine was administered 

 (Hoar, Goy and Young, 1957). Pregnant 

 euthyroid animals responded to thyroxine 

 by delivering more living young than nor- 

 mal control pigs (Peterson, Webster, Rav- 

 ner and Young, 1952) . The extent to which 

 such effects are consequences of the reduc- 

 tion in appetite, metabolism, and absorp- 

 tion of food from the gut which are associ- 

 ated with hypothyroidism has not been 

 determined. 



Hyperthyroidism, on the other hand, will 

 increase the appetite and enhance absorp- 

 tion of food from the gut, as the increased 

 metabolism requires more calories, minerals, 

 vitamins, choline, and methionine. In adult 

 rats hyperthyroidism induces a marked loss 

 in body fat and accelerates protein catab- 

 olism. The two effects, if unchecked, result 

 in loss of body weight and death. In im- 

 mature males hyperthyroidism slows gain 

 in body weight, retards testis growth and 

 maturation, and abolishes androgen secre- 

 tion (Table 12.2). Altering dietary protein 

 in adult animals failed to modify the thy- 

 roid hormone effects, thereby suggesting 



TABLE 12.2 



Effects of diet and thyroid {0.2 'per 



cent) on immature rats 



(From J. H. Leathem, in Recent Progress in 



the Endocrinology of Reproduction, Academic 



Press, Inc., New York, 1959.) 



that the metabolic demands of other tissues 

 were making increased withdrawals from 

 the metabolic pool of nitrogen and thus 

 hindering testis growth. In euthyroid rats, a 

 6 per cent protein diet will permit testis 

 growth in the absence of a gain in body 

 weight, but hyperthyroidism prevents this 

 preferential effect. Although Moore (1939) 

 considered the effect of thyroid hormone on 

 reproduction as possibly due to general body 

 emaciation, the testis seems to be less re- 

 sponsive than the body as a whole. Adult 

 rats fed 0.2 per cent desiccated thyroid 

 exhibited no correlation between loss of 

 body weight, change in testis weight, or 

 protein composition of testis at two levels 

 of casein and lactalbumin (Leathem, 

 1959b) . The testes were seemingly not in- 

 fluenced by the metabolic nitrogen changes 

 which caused a loss in carcass nitrogen 

 and an associated increase in kidney and 

 heart nitrogen. 



The mechanism of thyroid hormone ac- 

 tion on reproduction is far from clear. As 

 we have noted, a part of its action may be 

 through the regulation of nutritional proc- 

 esses. Thyroid function is influenced by 

 the biologic value of the dietary protein 

 (Leathem, 1958a) and the specifie amino 

 acids fed (Samuels, 1953). In turn, an 

 altered thyroid function will influence the 

 nitrogen contributions to the metabolic pool 

 by reducing appetite and absorption from 



