WATERTOWN, Mass., Oct. 25 (UPI) -- Testosterone levels in U.S. men have been declining steadily over the past two decades, say U.S. researchers.
The reasons for the decline are unclear; the study suggests that neither aging nor changes in certain health factors, such as obesity or smoking, can completely explain the phenomenon, according to study leader Thomas G. Travison of the New England Research Institutes in Watertown, Mass.
"Male serum testosterone levels appear to vary by generation, even after age is taken into account," said Travison. "In 1988, men who were 50 years old had higher serum testosterone concentrations than did comparable 50-year-old men in 1996. This suggests that some factor other than age may be contributing to the observed declines in testosterone over time."
The researchers analyzed data to compare men of similar ages during each phase of the study. For men 65 to 69 years of age in this study, average total testosterone levels fell from 503 ng/dL (nanograms/deciliter) in 1988 to 423 ng/dL in 2003. A normal, healthy adult male usually has blood total testosterone concentrations that range anywhere from 300-1,000 ng/dL.
The findings are published in the Journal of Clinical Endocrinology & Metabolism.
