It's been over a week now since we met Caitlyn Jenner, and her introduction has brought a multitude of transgender issues into the spotlight, including the physical changes that can accompany a transition. This new Reactions video from the American Chemical Society (below) is here to explain how hormone-replacement therapy fits in. We usually think of hormone-replacement therapy in the context of menopause, but hormones can be a huge part of transitioning as well. For starters, everyone has both estrogen and testosterone floating around — and these are some potent chemicals. Testosterone is involved in powering our sex drive and our sense of general well-being, and it also builds muscle. The testes produce about 10 times as much testosterone than the ovaries, and they create it about 20 times faster. So, hormone-replacement therapy is about getting the right balance of hormones in the body rather than substituting one for another. The effect is kind of like a second puberty. But, this time, it usually only takes about one or two years to complete. During this process, people will develop the secondary sex characteristics of the sex they're transitioning to. For instance, when transitioning from female to male, people are given testosterone as a topical gel or a shot; this causes the voice to deepen, facial hair to grow, and muscle to develop. This also lowers the amount of estrogen in the body, reducing breast size and causing the menstrual cycle to stop. And, when transitioning from male to female, people are given antiandrogens to block the effects of testosterone and lower the hormone's levels overall. This causes a reduction in male secondary sex characteristics. After taking estrogen, female characteristics will develop instead. Taking on physical sex characteristics is just one part of the complex transition process, but for many people it's a big one. And, because these hormones are responsible for so many things in our bodies, the decision to undergo hormone-replacement therapy should always be made with the help of a doctor. Because finding the right path to the "real" you is just as important as getting there.