When you add too much testosterone artificially, your body starts lowering production of your natural testosterone, because it thinks it’s over-producing. This is why testicles shrink on steroids—the body is shutting down hormone production there—and why fertility can drop dramatically. And confusingly, the overabundance of testosterone can actually prompt the growth of breasts. Bodies naturally convert some testosterone to estrogen. When you have a normal amount of hormones in your body, that’s not a problem. But if you jack up the T-levels, you also raise the amount of estrogen in the bloodstream. Ergo, breasts.