Testosterone is an androgen, or male hormone. While it's produced by both sexes--albeit in much larger amounts by males than by females--its most visible and familiar effects show up in men. Male fetuses begin producing the hormone at just a few weeks of gestational age, and boys increase their testosterone production during puberty, at which time it directs sexual development.
Developmental Functions
The earliest function of testosterone is to direct prenatal development. Male fetuses start producing testosterone early in gestation; it's made by the testes, and circulates throughout the body. In her book "Human Physiology," Dr. Lauralee Sherwood explains that early in development, both male and female fetuses look the same. Once the testes of a male fetus begin to synthesize testosterone, however, the female reproductive ducts die, while male reproductive ducts develop. Formerly undifferentiated external genitals become a penis and scrotum. The testosterone even has an effect on the brain of an unborn male baby; it causes the communication center of the brain to atrophy slightly relative to that of a female baby, and increases the size and activity of the aggression center.
Physical Effects
During puberty, testosterone has further physical effects upon boys. It causes the skin to produce more sweat glands and grow more body hair, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." It increases bone density and musculature, giving men heavier, thicker bodies than women. It thickens the tissue of the larynx, or voice box, leading to deepening of a man's voice. It also causes the testes to produce sperm, making a male reproductively capable. In short, during puberty, testosterone leads to development of what are called the masculine secondary sex characteristics, where primary sex characteristics are the testes and sex organs.
Psychological Functions
Testosterone doesn't just affect bodies; it also affects minds. In men, increased testosterone levels during puberty further increase aggression, as well as leading to sexual behavior and response. Interestingly enough, sex drive in women is also testosterone-dependent; in their book, "Biochemistry," Drs. Mary Campbell and Shawn Farrell explain that women also produce small amounts of testosterone. Concentrations aren't high enough under normal circumstances to produce male physiological or psychological characteristics, however. In fact, some physicians prescribe testosterone patches to menopausal women struggling with decreased sex drive.
References
- "Anatomy and Physiology;" Gary Thibodeau, Ph.D.; 2007
- "Human Physiology;" Lauralee Sherwood, Ph.D.; 2004
- "Biochemistry;" Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
