Low testosterone not normal part of aging HOUSTON, June 24 (UPI) -- A drop in testosterone levels in men as they age is more likely to result from smoking, obesity and depression -- not aging, researchers in Australia say. Dr. Gary Wittert, professor of medicine at the University of Adelaide in Adelaide in Australia, and colleagues analyzed testosterone levels in more than 1,500 men who had measurements taken at two clinic visits five years apart. Men who had abnormal values, were taking medications or had medical conditions known to affect hormones, were excluded. The men included in the analysis involved 1,382 men -- ages 35 to 80 years, with an average age of 54. The study found, on average, testosterone levels did not decline significantly over five years; rather, they decreased less than 1 percent each year. 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 in a statement. While stopping smoking may be a cause of a slight decrease in testosterone, the benefit of quitting smoking is huge. Declining testosterone levels are not an inevitable part of the aging process, as many people think, Wittert added. The findings are scheduled to be presented at the Endocrine Society's 94th annual meeting in Houston.