Responsible for the sexual behavior and characteristics of males, the male hormone testosterone is produced by the testes in relatively constant amounts throughout the life of an adult male. Occasionally, supplementation with testosterone or abnormalities in a man's physiology cause hormone levels to increase, which can lead to a number of symptoms, some of which are physical and some of which are psychological.
Testosterone increases the amount of oil produced by glands in the skin, explains Dr. Lauralee Sherwood in her book "Human Physiology." This is the reason that teenage males, upon entering puberty, begin to sweat more and notice their faces become oilier. Oily skin also increases the likelihood of acne, since oil can clog pores in the skin. If bacteria become trapped in clogged pores, they start to reproduce, which causes inflammation and redness. White blood cells attempt to fight the infection, leading to pus accumulation. Men who have high levels of testosterone experience oilier skin and more acne than those with more normal levels.
Testosterone has several effects upon the male brain. Early in prenatal development, the hormone causes the aggression center of the brain to become larger and more active, explain Drs. Michael Roizen and Mehmet Oz in their book "You: Having A Baby." Later, testosterone exacerbates this effect, causing men to experience and display more aggression than women or prepubescent boys. Increased testosterone levels increase male aggression. This leads to the phenomenon of "roid rage" that accompanies use of testosterone-like anabolic steroids, sometimes used by body builders to increase muscle mass.
Paradoxically, while testosterone is partially responsible for sperm production, in very high levels, it actually decreases fertility. This is because the male body attempts to maintain homeostasis, or stability, with regard to hormone levels, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." If testosterone levels get too high, they reduce levels of other hormones released by the hypothalamus and anterior pituitary, both of which are brain structures. When brain hormone levels decrease, they not only decrease levels of testosterone produced by the testes, they also decrease levels of other hormones that are required to initiate sperm production. As such, men with abnormal testosterone production or those taking testosterone-mimicking hormone supplements may decrease their sperm counts.