THE RELATION OF TEMPERATURE AND THE THYROID TO MAMMALIAN REPRODUCTIVE PHYSIOLOGY

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Abstract
In rams, sterility occurs during the hot summer months. This might be due to the direct effect of high temp. upon the testes. However, results demonstrate that the high temp. affects the thyroid gland which in turn indirectly affects the reproductive organs. During periods of high temp., there is a decrease in total spermatozoa/ejaculate, a lowered volume, and a greater percentage of morphologically and physiologically abnormal cells in the semen of rams. Injected or fed thyroxin during this period of high temp. causes the production of more spermatozoa with a lower percentage of abnormal cells. In the fall (normal breeding season) reproductive activity is at its peak. Creating a hypothyroid state by feeding thiouracil practically eliminates spermatogenesis and causes the production of a greater percentage of abnormal cells. These results are similar to those induced by high summer temps. Thyroxin, given to animals with low fertility as a result of thiouracil admn., restored spermatogenesis to its normal level. Since the temps. causing lowered fertility are below the body temp. of the ram, it is apparent that temp. is not acting directly upon the testes. Further, the results obtained during the summer period with thyroxin and during the fall period with thiouracil and thyroxin indicate that the high temps. cause a hypothyroid condition which is responsible for the reduction in spermatogenesis.