People who are transitioning genders want to look like the sex they’re transitioning to. Hormone therapy can help with that, since the characteristics that help us distinguish men and women by sight — breasts, relative muscle mass, the shape of the face (to an extent), and so forth — are created by the sex hormones our gonads start secreting at puberty.
Both sexes make testosterone and estrogen, but in different proportions: men have more testosterone, women more estrogen. The latest Reactions video from the American Chemical Society explains how readjusting the proportion of these hormones in their bodies helps trans men and women start to shift the way they look.
This post originally appeared on Throb, Gizmodo's blog of all things sex