Testosterone is an important body hormone that regulates the sex drive and male sexual characteristics. Like many hormones in the body, its release is regulated through several mechanisms. Unlike women, whose hormone levels fluctuate each month, men's bodies maintain constant levels of testosterone throughout adult life. As such, the mechanisms that regulate the release of testosterone in the body work toward maintaining homeostasis, or an even concentration of testosterone in the blood.
Hypothalamic Factor
At the top of the chain of hormones responsible for the eventual release of testosterone is GnRH, or gonadotropin releasing hormone, which comes from the hypothalamus. Explains Dr. Lauralee Sherwood in her book "Human Physiology," the hypothalamus is near the center of the brain, and is responsible for controlling the release of many other hormones. Most hormones released in the body travel through the bloodstream, and bind to specific target organs. The hypothalamus, however, does not release GnRH directly into systemic circulation. Instead, it releases the hormone into a special vessel called a portal vein, which carries the hormone directly to its target organ, the pituitary gland.
Pituitary Factors
Once the pituitary gland receives a signal from the hypothalamus, it releases two of its own hormones, called LH and FSH. LH, or luteinizing hormone, stimulates Leydig cells in the testes to secrete and release testosterone, explains Dr. Neil Carlson in "Foundations of Physiological Psychology." The cells make testosterone from cholesterol, which is found in the bloodstream. Through a series of chemical reactions, Leydig cells modify cholesterol into the related compound testosterone. Specifically, LH turns on an enzyme responsible for making several key bonds in the synthesis of testosterone from cholesterol.
Other Factors
Dr. Carlson notes that FSH, or follicle-stimulating hormone, also plays a role in testosterone release to a certain extent. The larger role of FSH is to stimulate Sertoli cells in the testes to produce sperm, but as sperm develop, they make a hormone of their own, called inhibin. Inhibit acts as a testosterone regulator, and inhibits the release of GnRH from the hypothalamus. In this way, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology," testosterone levels stay constant. If they start to get too high, the testes produce many sperm, which produce plenty of inhibit. High levels of inhibin then decrease GnRH release, which eventually results in decreased testosterone production.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Foundations of Physiological Psychology"; Neil Carlson, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
