We can thank the Greeks for the name doctors apply to male hormones. Androgen comes from the words meaning "man-maker," and it's a well-chosen term. Testosterone is the most potent androgen, and it does make the man. It's responsible for the deep voice, increased muscle mass, and strong bones that characterize the gender, and it also stimulates the production of red blood cells by the bone marrow.

In addition, testosterone has crucial, if incompletely understood, effects on male behavior. It contributes to aggression, and it's essential for the libido or sex drive, as well as for normal erections and sexual performance. Testosterone stimulates the growth of the genitals at puberty, and it is one of the factors required for sperm production throughout adult life.

Finally, testosterone also acts on the liver. Normal amounts are harmless, but high doses can cause liver disease and boost the production of LDL ("bad") cholesterol while lowering the amount of HDL ("good") cholesterol.

Although testosterone acts directly on many tissues, some of its least desirable effects do not occur until it is converted into another androgen, dihydrotestosterone (DHT). DHT acts on the skin, sometimes producing acne, and on the hair follicles, putting hair on the chest but often taking it off the scalp. Male-pattern baldness (androgenic alopecia) is one thing, prostate disease quite another — but DHT also stimulates the growth of prostate cells, producing normal growth in adolescence but contributing to benign prostatic hyperplasia (BPH) in many older men.

Scientists have taken advantage of the link between male pattern baldness and BPH to develop a single medication for both conditions. Finasteride blocks the conversion of testosterone to DHT; when taken in a 5-mg dose (Proscar), it helps some men with BPH, and in a 1-mg dose (Propecia), it helps some men with androgenic alopecia. Another drug, dutasteride (Avodart), has a similar effect on BPH but is not approved for baldness.

Is there another dark side to the DHT connection? Since DHT drives both hair loss and the growth of prostate cells, do men with androgenic alopecia have an increased risk of prostate cancer? Perhaps, according to scientists in Australia. They evaluated 1,446 men who were diagnosed with moderate to high-grade prostate cancer before age 70 and compared them with 1,390 men of the same age who were free of the disease. Even in the era of molecular biology, the research tool was simplicity itself. The researchers looked at each man's scalp, then used sophisticated statistical methods to see if there was a link between hair loss and prostate cancer. They found that men with bald spots at the top of their heads (vertex baldness) were one and a half times more likely to have prostate cancer than those without bald spots. The association was particularly strong for men who were diagnosed with high-grade prostate cancer at 60–69 years of age. In contrast, there was no link between a receding hairline (frontal baldness) and cancer. Although it may seem far-fetched, there are precedents for an association between vertex baldness and disease in men. Harvard's Physicians' Health Study found that men with bald spots were more likely to develop coronary artery disease than men with full heads of hair. Mild vertex baldness was linked to a 23% increase, moderate baldness to a 32% rise, and severe baldness to a 36% increase in risk. As in the Australian study of prostate cancer, frontal baldness was not associated with risk.

Although testosterone and DHT are the leading suspects, doctors don't know what accounts for the apparent associations between vertex baldness and prostate cancer and heart disease. Although explanations are on the thin side, there is no reason to think that hair loss itself is harmful to the prostate or heart — though it may take a toll on some men's self-image.

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