Tebbens M, Heijboer AC, T’Sjoen G, et al. - Findings showed that alteration in fat percentage and breast development in trans women were not related with estrone levels nor with administration route. Thus, measurement of estrone levels does not have a place in the monitoring of feminization in trans women.

• This prospective cohort study, with one year follow-up, included 212 adult trans women (median age 25 years) initiating hormone treatment between July 2017 and December 2019.

• Following 12 months of hormone therapy, estrone level was 187pmol/L and 1516pmol/L in transdermal and in oral estradiol users, respectively.

• In transdermal and in oral estradiol users, an increase in fat percentage by 1.2% (IQR 0.3 – 4.8) and by 4.6% (IQR 2.5 – 5.9) was observed, respectively.

• This was not related to estrone levels in transdermal (+4.4% per 100pmol/L increase in estrone concentration) nor in oral estradiol users (-0.7%).

• An increase in breast volume by 69ml (IQR 58 – 134) and by 62ml (IQR 32 – 95) was evident in transdermal and oral estradiol users, respectively.

• This was not related to estrone levels in transdermal (+ 14% per 100pmol/L increase in estrone concentration) nor oral estradiol users (+ 11%).