Thyroid Hormone Regulation of the Pulsatile Discharges of Luteinizing Hormone in Ovariectomized Rats

The pulsatile discharges of luteinizing hormone (LH) were characterized in ovariectomized rats in the presence or absence of thyroid hormone. LH secretion in ovariectomized rats with intact thyroid glands and thyroidectomized-ovariectomized rats receiving daily physiological doses of thyroxine (2 mug/100 g BW/day for 8 days) showed equivalent periodic discharges with frequencies between 15 and 45 min. Though the frequency of the plasma LH rhythm in untreated athyroid-ovariectomized rats was normal, the maximum and minimum concentrations were 2- to 3-fold higher than those of euthyroid-ovariectomized animals. On the other hand, treatment of athyroid-ovariectomized rats with a daily hyperthyroid dose of thyroxine (20 mug/100 g BW) for 8 days, attenuated pulsatile discharges of LH. The LH measured in the sera of these animals each gave dose-response curves by radioimmunoassay which were identical to the authentic rat LH reference preparation. Furthermore, neither the molecular profile nor the metabolic clearance rate of LH was affected by alterations in thyroid status. These results suggest that altered thyroid status does not influence the synthesis and metabolism of LH but does exert a profound effect on the secretion of this hormone by presumably acting directly on the hypothalamo-pituitary axis.