Testosterone: A "male hormone" -- a sex hormone produced by the testes that encourages the development of male sexual characteristics, stimulates the activity of the male secondary sex characteristics, and prevents changes in them following castration. Chemically, testosterone is 17-beta-hydroxy-4-androstene-3-one. Testosterone is the most potent of the naturally occurring androgens. The androgens cause the development of male sex characteristics, such as a deep voice and a beard; they also strengthen muscle tone and bone mass. High levels of testosterone appear to promote good health in men, for example, lowering the risks of high blood pressure and heart attack. High testosterone levels also correlate with risky behavior, however, including increased aggressiveness and smoking, which may cancel out these health benefits. Testosterone may be given to treat medical conditions, including female (but not male) breast cancer , hypogonadism (low gonadal function) in the male, cryptorchism (nondescent of the testis into the scrotum), and menorrhagia (irregular periods).
For guys without hypogonadism, a wait-and-see approach may make the most sense. "We just don't have the data to prove that raising testosterone in men who already have normal levels confers any real benefit," says Dr. Robert Mordkin , a urologist in Arlington, Virginia. Furthermore, Mordkin adds, "Natural waxes and wanes of testosterone as we age may have some not yet entirely understood protective effects." In addition to making us better fathers, as the Northwestern study infers, declining testosterone "may lower our vulnerability to prostate cancer at a time in our lives when we're at the greatest risk for it." That said, if you have classic symptoms of hypogonadism – low libido, fatigue, erectile dysfunction – "it's probably reasonable to check your testosterone. And for those guys whose blood levels come under definitions of low testosterone, it's probably reasonable to try the therapy for a while. But that's very different from saying testosterone is the fountain of youth."