Hormone therapy is used by transsexuals to change some secondary sex characteristics, including breast size, weight distribution, and hair growth. Like most aspects of human chemistry, the endocrine system - which controls the body's production and balance of hormones, including sex hormones - is still, at best, only loosely understood. However, one thing is for certain, hormones represent some of the strongest and subtlest chemicals produced by the body and should not be 'messed-with' without extreme care. Always consult a physician before using, or contemplating the use of, sex hormones on your body.