The Role of Estrone in Feminizing Hormone Treatment

CONTEXT: In trans women, hormone treatment induces feminization; however, the degree of feminization varies from person to person. A possible contributing factor could be estrone, a weak estrogen that interferes with the estrogen receptor. OBJECTIVE: We assessed whether estrone is involved in feminization induced by hormone treatment. METHODS: This prospective cohort study, with follow-up of 1 year, included 212 adult trans women at a gender identity clinic, who were starting gender-affirming hormone treatment between July 2017 and December 2019, median age 25 years. Change in fat percentage and breast development were assessed. RESULTS: After 12 months of hormone treatment, estrone concentration was 187 pmol/L (95% CI, 153-220) in transdermal and 1516 pmol/L (95% CI, 1284-1748) in oral estradiol users. Fat percentage increased by 1.2% (interquartile range [IQR], 0.3-4.8) in transdermal and 4.6% (IQR, 2.5-5.9) in oral estradiol users. This was not associated with estrone concentrations in transdermal (+4.4% (95% CI, −4.0 to 13) per 100 pmol/L increase in estrone concentration) nor in oral estradiol users (−0.7% [95% CI, −1.7 to 0.3]). Breast volume increased by 69 mL (IQR, 58-134) in transdermal and 62 mL (IQR, 32-95) in oral estradiol users. This was not associated with estrone concentrations in transdermal (+14% [95% CI, −49 to 156] per 100 pmol/L increase in estrone concentration) nor oral estradiol users (+11% [95% CI −14 to 43]). CONCLUSIONS: Change in fat percentage and breast development in trans women were not associated with estrone concentrations nor with administration route. Therefore, measurement of estrone concentrations does not have a place in the monitoring of feminization in trans women.