Cyproterone acetate (cypro) and spironolactone (spiro) are two of the most commonly prescribed anti-androgens in Australia.
For trans women and nonbinary people who haven’t had surgery to remove their testicles, these medications lower testosterone to levels typically seen in cis women.
Although both drugs reduce testosterone in the body, they work in very different ways – and surprisingly few studies have directly compared their effects.
This lack of research has led doctors to wonder if they might have different feminising effects, or if one might work better than the other for particular transition outcomes.
The search for better breast growth
As part of his PhD research looking into spiro and cypro, award-winning Adelaide endocrinologist Lachlan Angus and his team have recently published a paper comparing the two in terms of their effect on breast development.
One of the more common feminising effects trans people seek is breast growth. Breast development is often one of the earliest changes experienced after starting feminising hormone therapy, but it’s uncommon for trans people to achieve a substantial size without surgery.
As a result, some trans people have experimented with different drugs and techniques to enhance breast development, including progesterone. This hormone is responsible for changes in the mammary glands during puberty and pregnancy, among many other things, and some trans people report that it has helped them achieve larger, shapelier, or more sensitive breasts.
This study hypothesised that since cypro can mimic progesterone in the body, it might be better than spiro for this particular result.
Putting them to the chest
The study recruited 63 trans women and nonbinary people who had recently started taking estrogen.
They were assigned into random groups and given typical doses of each antiandrogen – one group given 12.5mg of cyproterone acetate and the other 100mg of spironolactone.
Breast size was measured at the beginning of the trial, at 3 months, and at 6 months. Measurements included both breast-chest distance (the difference between underbust and overbust) and volume. For this, the researchers used the Breast Idea Volume Estimator (BIVE), a web application developed for plastic surgeons to estimate breast volume.
This application is commonly used by surgeons to assess the breast volumes of cis women, but it had never been used for trans women. The study was the first to compare BIVE estimates with a more accurate 3D scan, confirming that it can measure trans women’s busts accurately and pioneering its use for trans patients.
It’s a bust
After 6 months, there was no statistically significant difference between the group taking cypro and the group taking spiro.
However, there was a huge difference between the participants themselves, with cup sizes ranging from AAA to B, no matter which antiandrogen they were taking. Overall, 59% of participants reported being satisfied or very satisfied with their breast development. After 9 months, breast-chest distance seemed to plateau, though volume continued to increase.
Larger sizes were correlated with age and blood estrogen level – though other studies have not shown any correlation between breast size and estrogen level. Since cypro mimics progesterone in the body, and some trans people have reported that progesterone aids breast development, the researchers were surprised that cypro didn’t result in greater breast growth.
The study also found that cypro did a better job in suppressing testosterone, and contrary to anecdotes, was not associated with higher depression scores.
The breast is yet to come
Given that both antiandrogens performed similarly, it seems that some other mechanism than progesterone or testosterone suppression is responsible for breast size in trans people.
While this is great news for trans people who desire breast growth and need to decide between the two antiandrogens, it indicates more research is needed to understand what factors influence breast development in trans women and the best way to achieve these transition outcomes.
