THYROIDECTOMY ABOLISHES SEASONAL TESTICULAR CYCLES OF SOAY RAMS

The hypothesis that thyroid hormones are required for the expression o f normal cycles of reproductive activity was tested in Soay rams, a wi dely used subject for studies on seasonality. Six rams were thyroidect omized and six sham-operated in October, just before the expected peak of reproductive activity. All were then placed in long days (18 h lig ht and 6 h dark, 18L:6D) for 12 weeks and then transferred to short da ys (8L:16D). All animals exhibited an initial short-term increase in t esticular size and serum concentrations of follicle-stimulating hormon e and luteinizing hormone then, as expected, these parameters decrease d in sham-operated animals for the remainder of the time they were mai ntained on long days. Four weeks after transfer back to short days, go nadotrophin concentrations in the control animals were significantly r aised above the lowest level, and scrotal size increased after eight w eeks. By contrast, in thyroidectomized rams all parameters remained hi gh throughout the experiment. Thyroidectomy therefore abolished season al changes in reproductive activity, indicating a requirement for thyr oid hormones for the normal expression of such patterns. It is unlikel y that this was related to any generalized metabolic disturbances caus ed by thyroidectomy, as body masses were similar in both groups of ani mals throughout the experiment.