Adding progesterone to gender-affirming hormone therapy improves breast growth for transgender people

The addition of the hormone progesterone to gender-affirming hormone therapy leads to increased breast growth for transgender people following feminizing hormone therapy. This is demonstrated by an Amsterdam UMC-led trial among 90 participants and these results are presented today at the European Professional Association for Transgender Health (EPATH) annual congress in Hamburg.

Our results show that progesterone is safe and effective for transgender people. We're now able to prescribe it, in a trial setting, for those who have been taking oestradiol for at least year. We hope that our findings lead to better hormone treatments for transgender individuals."

Koen Dreijerink, endocrinologist at Amsterdam UMC

Gender-affirming hormone therapy helps an individual's body better align with their gender identity. In the case of feminizing hormone therapy this involves blocking the action of testosterone and the addition of estradiol. Traditionally, any breast growth is then limited, leading many transgender people to ultimately opt for breast augmentation surgery.

Breast growth

Alongside estradiol, progesterone is one of the two key female sex hormones. Progesterone is known in cis women to also cause breast growth. However, progesterone has not been prescribed in transgender people due to a lack of evidence of its effectiveness and safety. In order to gain more information of the effect on breast volume as well as the safety of progesterone, Dreijerink and his colleagues conducted a randomly controlled trail between 2021 and 2024.

"Among our 90 participants we repeatedly used 3D-scanning techniques to measure breast volume and saw up to an increase of 37%. Crucially, we also saw that the study participants were more satisfied with the size, shape and the growth of their breasts," adds Raya Geels, PhD candidate at Amsterdam UMC and the study's first author.

The largest increase was seen in the group who also increased their estradiol dosage with some frequent side effects such as short-lasting tiredness, breast and nipple sensitivity and mood swings.

Participants in the study used progesterone for a year. "The reason we're moving forward with prescribing this in a research setting is to learn about the long-term effects and side-effects, for example we know that progesterone causes drowsiness so we advised our participants to take it prior to sleeping" adds Dreijerink. "It's important that we keep learning about the effects of gender affirming hormone therapy".