Testosterone Biology

Testosterone is the male sex hormone secreted primarily by the testes. Testosterone plays an important role in male development, resulting in enormous effects on the body during puberty and adulthood.

How Testosterone Works

Testosterone is derived from cholesterol. Like all hormones, testosterone is a chemical that is produced within a gland and secreted so it can travel (usually via the bloodstream) to other areas of the body, where it eventually comes in contact with its target cells. When the body produces testosterone, it targets tissues in the bloodstream, and travels through the blood, eventually binding to a plasma protein called sex hormone binding globulin(SHBG). The role of a hormone is to act as a messenger to the target cells. In the case of testosterone, the target cells are found in many different areas of the body, from the gonads to the brain. When these cells receive the testosterone signal, they reprogram which genes are turned on and which one are turned off.

Testosterone is either converted to estradiol, or reduced to DHT — two chemical reactions that cause the effects of testosterone.

The Role of Testosterone in Development

Testosterone induces changes in gene expression in its target cells. These changes can have a wide range of effects on the cells, depending on several factors, including the cell type. During puberty in males, these effects include:

  • Changes in vocal chord structure (result in a deeper voice)
  • Development of sex drive
  • Growth of body and facial hair
  • Induction of bone and muscle growth
  • Production of sperm.

The increase in testosterone levels that occurs in boys during early adolescence is the key signal to the body to undergo these physical and mental changes. This additional testosterone is largely produced by the testes.

During puberty and adulthood, women produce much less testosterone than men, but it is still required for normal development. Testosterone is secreted by the ovaries and the adrenal glands in females.

The Role of Testosterone in Adults

Although male sexual development is heavily reliant on testosterone, effects of this hormone can also be seen throughout adulthood in both sexes.

In adult men, testosterone appears to be important for maintenance of muscle tissue, bone density, cognitive function and sex drive. Testosterone levels in men generally start to decline after age 40. For most men, however, testosterone levels remain sufficiently high for the hormone to perform its needed functions. Men who are concerned that low testosterone levels are impacting their health can discuss with their doctors the possibility of supplemental testosterone pills to raise their testosterone levels. This treatment is not without risks, and some medical professionals have questioned the benefits of hormone replacement therapy in men with declining but still normal levels of testosterone.

Like men, adult women need testosterone for bone and muscle health, and possibly for sex drive. After menopause, women also experience an age-related decline in testosterone levels.


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