Effect of estrogens on prolactin secretion in transsexual subjects.

The effect of estrogens on the secretion of prolactin in 8 different groups of transsexual subjects was studied. Two different types of estrogens, estradiol or its conjugate and ethinyl estradiol, were used. Different doses and durations of exposure were employed. Plasma levels of prolactin and SHBG after estrogen exposure were compared with corresponding levels before treatment. Results showed that for estrogens to exert an enhancing effect on the secretion of prolactin, three factors needed to be considered: (i) the absolute concentration of estrogen, (ii) the duration of exposure, and (iii) whether levels of SHBG are sufficiently altered to change the concentration of free estrogen. It appears that there exist both time and dose thresholds for effective enhancement of prolactin secretion by estrogen. Estradiol and its conjugate are more likely to induce hyperprolactinemia than ethinyl estradiol. The reduced effect of the latter is probably related to its ability to induce large and rapid increases in SHBG binding which probably results in unaltered free estrogen concentration. In the light of this study, treatment of male transsexuals with high doses of estradiol in its conjugate forms must be viewed with caution.