Hormones such as estrogen, progesterone, and testosterone prepare the human body for reproduction and stimulate the development of sexual characteristics as an individual passes through puberty. In women, estrogen stimulates development of the breasts, ovaries, uterus, and the general shape of the body. The cells of the uterine lining are stimulated to grow and divide every month as part of the woman's menstrual cycle. If a pregnancy occurs, progesterone, along with estrogen, helps maintain the uterine lining to support development of the fetus. In men, testosterone stimulates development of the testes, sperm production, and growth of facial hair and musculature.
The sex hormones exert their varied effects by promoting cell growth and cell division. As long as the cells in the target organs are healthy, with checkpoint monitors intact, there is no problem. But if a crucial gene or set of genes is defective, and the cell is being bombarded with signals to proliferate, the situation can become serious very quickly. This is the reason why cancers of the breast, uterus, ovaries, and prostate are so common and so dangerous. Cancers such as these are a natural consequence of reproductive physiology, and treatments often try to reduce the levels of sex hormones to withdraw the stimulus to the tumor cells. As discussed more fully in the next chapter, this approach is not a pleasant experience, involving as it often does the removal of the ovaries, to eliminate the synthesis of estrogen, or the testes, to block production of testosterone.
The relationship between steroid hormones and cancer production introduces serious concerns regarding hormone replacement therapy, particularly for women, to minimize bone loss in the elderly. Hormone replacement therapy can alleviate some of the symptoms of osteoporosis in men and women, but it may do so at the risk of developing cancer. Similar concerns exist in the sporting world, where steroids are used to enhance performance. While such treatments seem to be effective, the athlete who uses hormone supplements will likely pay a heavy price for it in later life by developing prostate, breast, or uterine cancer.
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