Testosterone is a steroid sex hormone that is produced by the testes and the adrenal gland and is responsible for the development of the male reproductive tissues. Testosterone also promotes muscle, bone and hair growth. High levels of testosterone are usually a good indicator of overall health in men. The hormone has been shown to decrease blood pressure and reduce the risks of a heart attack. Various organs of the endocrine system are involved in the production and regulation of testosterone.
Similar to other steroid hormones, testosterone is synthesized in a metabolic pathway that originates with the compound, cholesterol. The National Institutes of Health explains that the majority of testosterone is produced in the male testes; smaller amounts are secreted from the adrenal glands. In women, testosterone is produced in the ovaries and the placenta. Levels of testosterone in women is typically much lower than the testosterone levels in men.
In males, the Leydig cells in the testes are responsible for the production of testosterone. As explained by F. J. Hayes and colleagues in "The Journal of Clinical Endocrinology and Metabolism" in 2001, hormones from the pituitary gland, luteinizing hormone, or LH, and follicle-stimulating hormone, or FSH, regulate the activity of the Leydig cells and can alter the amount of testosterone that is produced. When testosterone levels are low, the hypothalamus releases a hormone known as GnRH, or gonadotropin-releasing hormone. GnRH stimulates the pituitary to release LH and FSH, which stimulate the Leydig cells and increase testosterone production. Increasing levels of testosterone cause the hypothalamus to decrease the production and release of GnRH. Without this hormone, LH and FSH will no longer be released. Without these two pituitary hormones, the Leydig cells will not be stimulated and testosterone production will decrease. This entire process is known as a negative feedback loop.
A blood test is an easy way to assess blood levels of testosterone. An analysis of testosterone levels is typically needed to assess early or late puberty in boys, infertility in men or excess hair growth and irregular menstrual cycles in women. According to the National Institutes of Health, for a male, a normal testosterone level is between 300 to 1,200 ng/dL, nanograms per deciliter. For a female, testosterone levels should fall between 30 to 95 ng/dL.
Low Testosterone Levels
As explained by R. N. Baumgartner and P. J. Garry in their study published in "Metabolism" in 1997, testosterone levels tend to decline after the age of 40. Levels drop by approximately 1 to 2 percent per year. This natural decrease is usually not a cause for clinical concern. However, some men do experience symptoms such as erectile dysfunction, low sex drive, mood swings and fatigue. Men with levels of testosterone lower than 300 ng/dL could be candidates for hormone replacement therapy. The benefits of such treatments are still under debate.
High Testosterone Levels
High testosterone levels are linked to good health in men. These individuals are less likely to have high blood pressure, be obese or be at risk for heart attacks.
- "The Journal of Clinical Endocrinology and Metabolism"; Differential Regulation of Gonadotropin Secretion by Testosterone in the Human Male: Absence of a Negative Feedback Effect of Testosterone on Follicle-Stimulating Hormone Secretion; F. J. Hayes et al.; January 2001
- National Institutes of Health; Medline Plus: Testosterone
- "Metabolism"; Longitudinal Changes in Testosterone, Luteinizing Hormone, and Follicle-Stimulating Hormone in Healthy Older Men; Richard N. Baumgartner and Phillip J. Garry; April 1997