Now that we have learned that testosterone plays an important role in maintaining sexual desire in both sexes, we might ask, How much testosterone is necessary to ensure normal sexual arousability? The answer to this question is complex and is influenced by several factors.
Testosterone in the bodies of both sexes comes in two forms: attached (bound) and unattached (free). About 95% of the testosterone circulating in a man’s blood is bound on a protein molecule (either albumin or globulin), where it is inactive or metaboli — cally ineffective. The remaining 5% is the unattached version of testosterone, which is metabolically active and influences male libido (Crenshaw, 1996; Donnelly & White, 2000). Comparable figures for women are 97 to 99% bound testosterone and only 1 to 3% free testosterone to produce effects on bodily tissues (Rako, 1996). The sum of free and bound testosterone in each man or woman is total testosterone. The normal range of total testosterone in the blood of a man is 300-1,200 ng/dL (nanograms per deciliter; a nanogram is one-billionth of a gram). In women the normal range of total testosterone is 20-50 ng/dL (Rako, 1996; Winters, 1999). The essential amount, or critical mass, of testosterone necessary for adequate functioning varies from person to person in both sexes. That women normally have much smaller amounts of testosterone than men do does not mean that women have lower or weaker sex drives than men. Rather, women’s body cells seem more sensitive to testosterone than men’s body cells. Therefore only a little testosterone is necessary to stimulate female libido (Bancroft, 2002; Crenshaw, 1996).
Too much testosterone can have adverse effects on both sexes. Excess testosterone supplements in men can cause a variety of problems, including disruption of natural hormone cycles, salt retention, fluid retention, and hair loss. Furthermore, although no evidence suggests that testosterone causes prostate cancer, excess testosterone can stimulate growth of preexisting prostate cancer (Jannini et al., 2011). In women excess testosterone can stimulate significant growth of facial and body hair, increase muscle mass, reduce breast size, and enlarge the clitoris (Kingsberg, 2002; Shah & Montoya, 2007). However, in most instances only the use of irresponsibly high doses of testosterone over a sustained period of time results in the development of adverse side effects in either sex (Rako, 1999). Furthermore, as we have seen, supplementary testosterone can help restore sexual desire to men and women with deficient levels of this libido hormone.
A normal level of total testosterone in either sex does not necessarily rule out a biological basis for a flagging sex drive, because the key hormonal component in libido— free testosterone—can be abnormally low even though the total testosterone level is within normal limits. Consequently, should you find yourself experiencing testosterone deficiency (see the next section), it is important that, as an informed consumer of health care, you have your free testosterone levels assessed in addition to your total testosterone level.
Finally, the rate at which testosterone production diminishes with aging differs markedly in men and women. As commonly happens, when a woman’s ovaries begin to shut down at menopause, her total testosterone may fall quickly in just a matter of months. For other women the onset of testosterone deficiency is more gradual, taking place over a period of several years (Kingsberg, 2002). (Women who have their ovaries surgically removed are more likely to experience an abrupt or precipitous loss of testosterone.) When a woman’s ovaries are no longer producing normal levels of testosterone, even though her adrenal glands continue to produce testosterone, their output also diminishes (Rako, 1999).
In contrast, in men the decline in testosterone with aging is usually much less precipitous. Testosterone production remains stable in most men until around age 40, after which it declines by 1 to 2% each year (Makinen et al., 2011).
The general signs of testosterone deficiency are similar in both sexes, even though they have a more rapid onset in women than in men. The most obvious symptoms of testosterone deficiency are listed in ■ Table 6.2.