Control of gonadotrophin secretion by steroid hormones in castrated male transsexuals. I. Effects of oestradiol infusion on plasma levels of follicle-stimulating hormone and luteinizing hormone.

Twenty-four infusions of oestradiol (E2) in graded doses ranging from 0--200 micrograms administered over a period of 7 hours were carried out in eleven healthy male transsexuals who had undergone sex reassignment at least 3 months previously. Plasma levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH) and E2 were analysed by radioimmunoassay, while sex hormone binding globulin (SHBG) was measured using 3H-testosterone as the saturating ligand. Infusion of 5--200 micrograms of E2 raised plasma E2 to levels ranging from 38.4--367.8 pg/ml which present 83%--800% of levels found in a control group of sixty-three normal males. SHBG capacity remained unchanged at all doses of E2 studied. No change in plasma levels of FSH and LH was observed in control infusions and infusion of 5 micrograms of E2. From 10 micrograms-200 micrograms, suppression of plasma levels of FSH was noted at the 5--7 hour period. The suppression increased up to 20 micrograms and thereafter the levels of FSH remained constant. On the other hand, the suppression of LH increased up to the highest E2 dose (200 micrograms) studied. Further, significant suppression of LH occurred earlier than the 5--7 hours as the dose of E2 increased. These observations are consistent with the conclusions that: (1) E2 plays a part in the regulation of secretion of FSH and LH in men; and (2) at doses higher than physiological, E2 exerts a differential effect on the secretion of FSH and LH.