In my last post about hormone gatekeeping I referenced an ongoing study on the effects of progesterone and oestrogen levels on feminisation in gender-affirming hormone therapy. While the full study report isn’t out yet some findings have been published publicly.

I’ll summarise the study protocol below, but the study has shown progesterone increases breast tissue volume and satisfaction with feminisation more than oestrogen alone does, and that increasing oestrogen levels also improves results.

The full trial protocol is in the trial announcement, but it divided participants in to six groups by oestrogen and progesterone dose. Half of the participants followed standard oestrogen dose for GAHT in The Netherlands, which targets 200-400 pmol/L of oestradiol, and the other half had the target doubled to 400-800 pmol/L. Those two groups were subdivided in to no progesterone, 200µg progesterone daily, and 400µg progesterone daily.

I’ll let the announcement speak for itself

“Among our 90 participants we repeatedly used 3D-scanning techniques to measure breast volume and saw up to an increase of 30%. Crucially, we also saw that the study participants were more satisfied with the size, shape and the growth of their breasts compared to participants who did not use progesterone,” 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 oestradiol dosage with some frequent side effects such as short-lasting tiredness, breast and nipple sensitivity and mood swings.

So this study suggests that higher levels of oestradiol and the incorporation of bio-identical progesterone are helpful and safe, at least over a year. The full text isn’t available in any journals I have access to yet, but I’ll be keeping an eye out.

While there were only 90 participants this is the largest and most rigorous study on the effects of oestradiol and progesterone levels in gender affirming treatment ever done. Hopefully this can start putting to rest the “it’s dangerous” and “there’s no evidence” arguments.