Control of gonadotrophin secretion by steroid hormones in male castrates: site of oestradiol action.

Four normal healthy castrated male transsexuals who were not on hormone therapy for at least 4 months volunteered for this study. Each subject received 3 infusions at weekly intervals--the first consisting of LHRH alone, the second of LHRH and oestradiol and the third of oestradiol alone. Venous blood samples were collected at 20-minute intervals 1 hour before the start and during the whole period of infusion. Plasma concentrations of FSH, LH and oestradiol were measured by radio-immunoassay. On an average, 100 micrograms of LHRH enhanced the area under the LH secretion curve by 177% and under the FSH curve by 68%. Oestradiol had a differential effect on the endogenous secretion of LH and FSH, suppressing them by 53% and 18%, respectively. Similarly, the LHRH-stimulated release of LH was significantly inhibited to a greater extent (42%) than that of FSH (23%, P less than 0.05; paired t test). Such an observation and in the light of our acceptance of the single releasing hormone theory, suggests that the differential actions of oestradiol on the secretions of FSH and LH are due to mechanisms residing in the pituitary.