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).
Other significant adverse effects of testosterone supplementation include acceleration of pre-existing prostate cancer growth in individuals who have undergone androgen deprivation; increased hematocrit , which can require venipuncture in order to treat; and, exacerbation of sleep apnea .  Adverse effects may also include minor side-effects such as acne and oily skin, as well as, significant hair loss and/or thinning of the hair, which may be prevented with 5-alpha reductase inhibitors ordinarily used for the treatment of benign prostatic hyperplasia , such as finasteride .  Exogenous testosterone may also cause suppression of spermatogenesis , leading to, in some cases, infertility.  It is recommended that physicians screen for prostate cancer with a digital rectal exam and prostate-specific antigen (PSA) level before starting therapy, and monitor PSA and hematocrit levels closely during therapy.