Effects of hyper- and hypothyroidism on serum LH and FSH levels in intact and gonadectomized male and female rats.

The effects of hypo- and hyper-thyroidism on serum LH and FSH were determined in both intact and castrated male and female rats. Thyro-parathyroidectomy (Tx) for 30 days in rats with intact gonads resulted in a significant reduction in serum LH and FSH, and also in a decrease in serum testosterone in males. Administration of 2.5 mug thyroxine (T4)/100 g BW to Tx rats of both sexes returned serum LH and FSH levels to those of intact rats, and in males also restored normal serum testosterone levels. Tx superimposed upon castration resulted in a significantly greater increase in serum LH and FSH than produced by castration alone. Administration of 2.5 mug T4/100 g body weight to castrate-Tx rats reduced serum LH and FSH values to those of castrate rats, whereas 10 mug T4/100 g BW evoked a further decrease in serum LH but no additional reduction in serum FSH. When both 2.5 mug T4/100 g BW and 2 mug estradiol benzoate were injected into Tx-ovariectomized rats, the decrease in serum LH and FSH was much greater than produced by T4 alone. These observations indicate that hypothyroidism results in decreased release of LH and FSH in rats with intact gonads, and in increased release of LH and FSH in castrate rats of both sexes. Administration of a replacement dose of T4 can restore LH and FSH release to normal in Tx rats with intact gonads, and to castration levels in Tx-castrate rats.