Testosterone is a hormone produced by the male testicles and, in smaller quantities, by the female ovaries. In the fetus, testosterone is responsible for the development of primary male sexual characteristics, particularly the testicles and penis. In adolescent males, testosterone is responsible for the development of secondary sexual characteristics such as facial and body hair, the development of musculature, deepening of the voice and even aggressive behavior. In both sexes it helps regulate potency and sexual desire. It is produced naturally throughout a person's lifetime.
Testosterone can be artificially synthesized. Treatment with testosterone is common for patients who suffer impotence or low sex drive due to a lack of natural production of the hormone. However, because of its ability to build muscle, it is often abused by body builders and athletes. Side effects of such abuse can be extremely aggressive behavior and the atrophy of the testicles. Exposure of children to testosterone can result in early puberty in both males and females.
In Act Your Age the symptoms of the patients are traced to one character's use of testosterone cream.