Purpose: Maintaining cross-sex hormone levels in the normal physiologic range for the desired gender is the cornerstone of transgender hormonal therapy, but there are limited data on how to achieve this. We investigated the effectiveness of oral estradiol therapy in achieving this goal. Methods: We analyzed data on all transgender females seen in our clinic since 2008 treated with oral estradiol. We looked at the success of achieving serum levels of testosterone and 17-β estradiol in the normal range on various doses of estradiol (with and without antiandrogens spironolactone and finasteride). Results: There was a positive correlation between estradiol dose and 17-β estradiol, but testosterone suppression was less well correlated. Over 70% achieved treatment goals (adequate 17-β estradiol levels and testosterone suppression) on 4 mg daily or more. Nearly a third of patients did not achieve adequate treatment goals on 6 or even 8 mg daily of estradiol. Spironolactone, but not finasteride, use was associated with impairment of obtaining desired 17-β estradiol levels. Spironolactone did not enhance testosterone suppression, and finasteride was associated with higher testosterone levels. Conclusions: Oral estradiol was effective in achieving desired serum levels of 17-β estradiol, but there was wide individual variability in the amount required. Oral estradiol alone was not infrequently unable to achieve adequate testosterone suppression. Spironolactone did not aid testosterone suppression and seemed to impair achievement of goal serum 17-β estradiol levels. Testosterone levels were higher with finasteride use. We recommend that transgender women receiving estradiol therapy have hormone levels monitored so that therapy can be individualized.
Keywords: finasteride; oral estradiol; spironolactone; testosterone suppression; transgender woman.