A new study finds that a drop in testosterone levels over time is more likely to result from a man's behavioral and health changes than by aging. The study results will be presented Monday at The Endocrine Society's 94th Annual Meeting in Houston.

"Declining testosterone levels are not an inevitable part of the aging process, as many people think," said study co-author Gary Wittert, MD, professor of medicine at the University of Adelaide in Adelaide, Australia. "Testosterone changes are largely explained by smoking behavior and changes in health status, particularly obesity and depression."

Many older men have low levels of the sex hormone testosterone, but the cause is not known. Few population-based studies have tracked changes in testosterone levels among the same men over time, as their study did, Wittert said.

In this study, supported by the National Health and Medical Research Council of Australia, the authors analyzed testosterone measurements in more than 1,500 men who had measurements taken at two clinic visits five years apart. All blood testosterone samples underwent testing at the same time for each time point, according to Wittert.

After the researchers excluded from the analysis any men who had abnormal lab values or who were taking medications or had medical conditions known to affect hormones, they included 1,382 men in the data analysis. Men ranged in age from 35 to 80 years, with an average age of 54.

On average, testosterone levels did not decline significantly over five years; rather, they decreased less than 1 percent each year, the authors reported. However, when the investigators analyzed the data by subgroups, they found that certain factors were linked to lower testosterone levels at five years than at the beginning of the study.

"Men who had declines in testosterone were more likely to be those who became obese, had stopped smoking or were depressed at either clinic visit," Wittert said. "While stopping smoking may be a cause of a slight decrease in testosterone, the benefit of quitting smoking is huge."

Past research has linked depression and low testosterone. This hormone is important for many bodily functions, including maintaining a healthy body composition, fertility and sex drive. "It is critical that doctors understand that declining testosterone levels are not a natural part of aging and that they are most likely due to health-related behaviors or health status itself," he said.

Unmarried men in the study had greater testosterone reductions than did married men. Wittert attributed this finding to past research showing that married men tend to be healthier and happier than unmarried men. "Also, regular sexual activity tends to increase testosterone," he explained.