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The exact causes of the age-related reduction in testosterone levels is not known; it is probably the result of a combination of factors, including increased body fat (and therefore increased aromatase activity), oxidative damage to tissues responsible for the production of testosterone, and declining levels of precursor molecules such as DHEA. The results of the decline, however, are strikingly apparent.
There is a clear relationship between low levels of testosterone and increased incidence of cardiovascular disease, particularly as testosterone level relates to metabolic syndrome (Dobrzycki S et al 2003; Hak AE et al 2002; Zhao SP et al 1998; Jeppesen LL et al 1996). Metabolic syndrome is the combination of abdominal obesity, high blood pressure, insulin resistance, and lipid disorders in the same person. This condition is associated with a high risk of cardiovascular disease. Studies have shown that testosterone administration (500 milligrams [mg] of intramuscular injections) in middle-aged, obese men was able to increase insulin sensitivity (Marin P et al 1992a). These results were confirmed in another study in which testosterone treatment led to reduced insulin resistance (Marin P et al 1992b). Later studies also showed that testosterone administration is helpful in the context of metabolic syndrome (Bhasin S 2003; Boyanov MA et al 2003).
When testosterone levels are measured, it is critical to determine the levels of both free and total testosterone to understand the cause of any observed symptoms of deficiency or excess (Pardridge WM 1986).
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