Factors That Affect Testosterone Levels
Testosterone, which is abundantly produced during puberty, is responsible for the development of secondary sexual characteristics and has profound effects throughout the body. Testosterone receptors are found in virtually all body tissues, so levels of testosterone affect the function of most organ systems. For example, testosterone causes growth of facial and body hair in skin cells, increased fiber size and strength in muscle cells, and maturation of the external genitalia. The effects of testosterone on the central nervous system are also well-known (Okun MS et al 2004). Testosterone governs such behaviors as aggression, risk taking, and territoriality. It is now recognized as an important factor in mitigating depression (King JA et al 2005).
As with all sex hormones, testosterone is part of a cascade that begins with cholesterol, the building block for hormones. Pregnenolone, the “master hormone,” is produced directly from cholesterol. In turn, dehydroepiandrosterone (DHEA; a precursor to testosterone), testosterone (and its metabolites), and estrogen (and its metabolites) are produced from pregnenolone. Both testosterone and estrogen are produced by enzymatic reactions from DHEA. This ubiquitous molecule is the steroid found in highest quantities in humans. Changes in the supply of DHEA (and/or changes in the levels of the enzymes that convert DHEA to the sex hormones) can have powerful effects on sex hormone–dependent systems.
As men age, a number of changes occur that reduce the testosterone level available to the body and that alter the ratio between testosterone and the chief female hormone, estrogen. This condition is now referred to as partial androgen deficiency of aging men (Harman SM 2005).
One of the most important factors that affect testosterone levels of aging men is an enzyme called aromatase, which is found in fat tissue. This enzyme is responsible for converting testosterone into estrogen, thus altering the ratio of estrogen to testosterone (Steiner MS et al 2003). Men who have excessive body fat, especially abdominal fat, are likely to have increased estrogen levels caused by aromatase activity and a dramatically increased estrogen level compared to testosterone. An increased estrogen level has been linked, in turn, to a host of disorders, including decreased insulin sensitivity and blood glucose problems. Some studies suggest that there is an association between a low testosterone level, insulin resistance, an elevated estrogen level, and increased body fat in aging men (Phillips GB 1993).
This relationship between low testosterone and obesity has been described as the hypogonadal/obesity cycle. In this cycle, a low testosterone level leads to an increase in abdominal fat, which leads to increased aromatase activity, which leads to further conversion of testosterone to estradiol, which further reduces testosterone and increases the tendency toward abdominal fat (Cohen PG 1999).
The effect of sex hormones on tissues is also affected by the level of sex hormone–binding globulin (SHBG). Sex hormones circulate in the bloodstream in very small quantities as free molecules. The bulk of sex hormones are bound to SHBG, which is a specialized carrier protein (Nankin HR et al 1986). Hormones bound to carrier molecules are inactive, so the amount of SHBG has an important impact on the degree to which tissues respond to sex hormone levels (Misao R et al 1999; Zmuda JM et al 1993; Dambe JE et al 1983; Van Look PF et al 1981). Nutritional status and the levels of other hormones are among the factors that determine levels of SHBG.
Aging men who have an androgen deficiency experience both an increase in aromatase activity and an elevation in SHBG production. The net result is to increase the ratio of estrogen to testosterone and lower the total testosterone level (Killinger DW et al 1987; Kley HK et al 1980a,b). Finally, it is important that aging men also strive for optimal liver function. The liver is responsible for removing excess estrogen and SHBG, so any compromise in liver function (such as that caused by heavy alcohol consumption, for example) can exacerbate hormonal imbalances.
