Control of gonadotrophin secretion by steroid hormones in castrated male transsexuals. II. Effects of androgens alone and in combination with oestradiol on the secretions of FSH and LH.

Twenty-nine infusions in twenty castrated male transsexual volunteers were carried out over a period of 7 h with subjects lying in the supine position. The effects of different doses of testosterone and its 5 alpha-reduced metabolites as well as the effect of testosterone in combination with oestradiol on gonadotrophin secretion were evaluated. Different and varying degrees of suppression of plasma levels of FSH and LH were observed. The infusions of 2.4 mg testosterone, 5 alpha-androstan-3 alpha-17 beta-diol (3 alpha-diol), 5 alpha-androstan-3 beta-17 beta-diol (3 beta-diol) but not dihydrotestosterone (DHT) caused significant suppression of LH. FSH, on the other hand was not significantly inhibited by the androgens at this rate. At higher doses all four androgens suppressed LH secretion significantly. FSH was similarly suppressed by the androgens except by DHT. A differential effect on FSH and LH secretions was noted with the combined regime of testosterone and oestradiol. The combined regime did not cause a significantly higher degree of FSH suppression compared with either 200 microgram of oestradiol or 12 mg of testosterone infused alone. The level of LH, however, was suppressed to a greater extent than either of the hormones when given alone. The inhibitory effect of testosterone demonstrated in this study could be due to the parent hormone or its 5 alpha-reduced metabolites. Pharmacological doses of testosterone could exert a greater degree of LH suppression through its conversion to oestradiol. It is likely that oestradiol and testosterone act on gonadotrophin secretion through different mechanisms and that they have an additive suppressive effect on the secretion of LH but not FSH. The potencies for the androgens to suppress gonadotrophin secretion can be ranked as: 3 alpha-diol = 3 beta-diol greater than testosterone greater than dihydrotestosterone.