Spironolactone

Spironolactoneis a steroidal [anti-androgen] used in feminizing hormone regimens. It the most frequently steroidal anti-androgen for feminizing HRT in the United States.

Testosterone levels
Spironolactone is commonly described as lowering testosterone levels. However, a 2014 study found that in 25 transgender women treated with spironolactone, testosterone levels did not decrease and there was in fact a trend towards statistically significantly increased levels of testosterone (130 ng/dL vs. 108 ng/dL). In contrast to spironolactone, estradiol treatment substantially suppressed testosterone levels.

Breast development
In a recent survey, transgender patients prescribed spironolactone were more likely to ask for breast augmentation than patents prescribed cyproterone.