Testosterone is an androgen, one of the male sex hormones synthesized from cholesterol in the Leydig cells, also called the interstitial cells of Leydig, in the testes. Leydig cells also synthesize and secrete the androgens androstenedione and dihydroepiandosterone (DHEA), but testosterone is the principal male sex hormone. Small amounts of testosterone are also secreted by the adrenal glands. The ovaries of women also produce testosterone.
Endocrine Regulation
The amount of testosterone created and secreted is dependent upon the amount of circulating luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These two hormones are secreted by the pituitary gland and their levels are controlled by the amount of gonadotropin releasing hormone (GnRH) released from the hypothalamus to act on the pituitary. This combination of hypothalamus, pituitary and gonads is called the HPG Axis. It acts as a self-regulating entity, constantly adjusting the levels of the different hormones in response to various physiologic and environmental factors. For example, high levels of inhibin or testosterone will act on the pituitary to reduce LH and FSH release and will act on the hypothalamus to reduce GnRH release, while low levels of circulating testosterone have the opposite effects.
Normal Testosterone Levels
The level of testosterone considered normal in adult males has an extremely wide range. Most healthy young adult males have levels between 5 and 15 ng/dl, measured as free testosterone. The strongest influence on one's level is simply age. Plasma levels of testosterone peak at an average of 30 years of age and are reduced by roughly 25 percent per decade thereafter.
Other Influences
Besides age, testosterone levels have been shown to be influenced by a whole host of factors. Time of day, sleep patterns, familial/relationship status, physical exercise, sexual activity, stress, diet and psychological status have all been demonstrated to significantly influence testosterone levels. The relationships between these factors and testosterone levels are complex and definitely not linear, but in general, adequate sleep, resistance exercise and high self-esteem are associated with higher levels, while marriage and fatherhood, a low-calorie and/or low-fat diet and stress are associated with lower levels.
Function
Testosterone is a steroid hormone, which means it is transported through the blood stream by a protein, crosses through cell membranes and acts by binding to an intracellular receptor. The resulting steroid receptor complex migrates to the nucleus of the cell where it initiates transcription of specific genes. For testosterone, these genes serve to promote the development of male sex characteristics during development and puberty (i.e., genitalia, larynx and hair growth). In adult males, testosterone helps to maintain those characteristics but also maintains spermatogenesis and exerts an important anabolic (muscle building) effect.
Testing
Testosterone is naturally occurring hormone that is essential for proper physiological function and is generally well-regulated by the human body. In other words, although levels vary somewhat between humans, you probably have the testosterone concentration you are supposed to have. However, if you suspect your testosterone level may be abnormal, your doctor can measure it with a simple blood test and regulate it with a variety of treatments.
References
- "Sports Medicine"; Hormonal responses and adaptations to resistance exercise and training; Kraemer WJ, Ratamess NA; 2005
- "Journal of Applied Physiology"; Testosterone and cortisol in relationship to dietary nutrients and resistance exercise; Volek, et al.; 1997
- "Sleep Medicine Reviews"; The effects of testosterone on sleep and sleep-disordered breathing in men: its bidirectional interaction with erectile function; Andersen ML, Tufik S; October 2008
- "Psychoneuroendocrinology"; Relationship status and testosterone in North American heterosexual and non-heterosexual men and women: cross-sectional and longitudinal data; van Anders SM, Watson NV; July, 2006
- "Psychological Science"; Submitting to defeat: social anxiety, dominance threat, and decrements in testosterone; Maner JK, Miller SL, Schmidt NB, Eckel LA; August 2008
