Testosterone is the principal androgen, or male sex hormone, although it appears in both genders. It is classified as an anabolic steroid because it will bulk up body tissues and encourage the retention of protein by the body. In both men and women, testosterone is produced in the gonads, and it is derived from cholesterol. In addition to occurring naturally in the body, this hormone is also used to treat certain medical conditions, and some athletes take it to bulk up for competition, although this practice is outlawed by most professional sports organizations.
In both genders, a testosterone production spike during early puberty helps to mature the body, encouraging the growth of body hair, sparking a growth spurt, and creating more oily skin and hair. In boys, the spike deepens the voice, causes the testicles to descend, and contributes to the development of other sex characteristics. Throughout the life of a man, his body will continue to produce testosterone, although the production rate will decline in old age.
Testosterone also contributes to physical differences between male and female brains. The male brain is actually larger, as are many other organs of the male body, because the hormone causes a greater growth rate. However, the corpus callosum, or connection between the hemispheres of the brain, is larger in females. The exact impact that this has on cognition and abilities is unclear, although numerous studies on the issue have been undertaken.
Therapeutically, testosterone is often used to treat conditions caused when not enough is naturally produced by the body. A blood test can be performed to see how much the body is creating, and if a doctor deems it necessary, he or she will prescribe therapeutic hormones. It is also used in the treatment of some cancers which have been shown to respond well to steroids, and in the treatment of women with gender dysmorphia to assist them in developing male sex characteristics while they transition.
Like other steroids, testosterone may be harmful in excess, especially when extra is taken without proper medical supervision; acne, highly oily skin, growth of some cancers, difficulty sleeping, and other complications are associated with abnormally high testosterone levels. However, a deficiency can also be harmful, and will have an impact on libido, energy, and maintenance of general health. Most human bodies are able to regulate hormone levels naturally, requiring no medical intervention. Some older men participate in androgen replacement therapy programs when their natural testosterone levels decline in order to maintain their younger state of health and vigor.