Men with lower levels of testosterone may be at increased risk of depression, a new study finds.

Researchers found that more than half of the men in the study who had lower levels of testosterone had a diagnosis of depression, or showed symptoms of the condition, while a quarter of participants were taking medication for the disease. The vast majority of male participants in the new George Washington University study also were found to be overweight or obese, and so for comparison, the researchers pointed to a recent survey of U.S. adults finding that 6 percent of those overweight or obese were depressed.

"Depression and/or depressive symptoms were present in 56 percent of the subjects," in the study, the authors concluded.

Produced primarily by the testicles, testosterone helps maintain a man's sperm production, sex drive, muscle strength and mass, bone density, and facial and body hair. Men who do not produce a "normal" amount of testosterone may be diagnosed with a condition called hypogonadism, but exactly what level should be considered normal is difficult to define, the authors wrote.

One reason for this is that blood levels of testosterone may be less important to a man's health than the effects of the hormone in muscle, bone, the brain and the reproductive organs, explained the authors. But even with blood tests, there is no level of testosterone that is universally accepted within the medical community as being too low.

Testosterone levels generally peak during adolescence and early adulthood. As men age, their testosterone levels gradually decline, typically by about 1 percent a year after age 30 or 40, according to the Mayo Clinic’s website. [5 Myths About the Male Body]

In the new study, the researchers probed the medical charts of 200 men with an average age of 48. All had been referred to an endocrinologist after a blood test indicated their testosterone levels were borderline low (between 200 and 350 ng/dL).

The researchers looked at the men's demographic data, medical histories, medication use, and symptoms of hypogonadism. They also looked at whether the men had been diagnosed with depression or if they took an anti-depressants, and all study participants who weren't diagnosed with depression or taking medications for the condition answered standardized test questions aimed at measuring their mood.

Analysis showed that the study participants had higher rates of obesity and lower rates of physical activity than their peers in the general population. Participants also suffered from erectile dysfunction, decreased libido, fewer morning erections, low energy and sleep disturbances. Rates of depression were 62 percent for study participants in their 20s and 30s, 65 percent for those in their 40s, 51 percent for those in their 50s and 45 percent for those age 60 and over.

"In an era where more and more men are being tested for '"Low T' — or lower levels of testosterone — there is very little data about the men who have borderline low testosterone levels," study researcher Dr. Michael Irwig, an associate professor of medicine and director of the Center for Andrology at the George Washington School of Medicine and Health Sciences in Washington, D.C., said in a statement. "We felt it important to explore the mental health of this population."

More research is still needed in this area, but doctors and other health care professionals "should recognize the high rates of depression and depressive symptoms in men referred for borderline testosterone levels," the authors wrote in their study.

Testosterone replacement therapy can improve the signs and symptoms of low testosterone in these men, the researchers said. In the decade ending in 2011, one commercial health insurance group in the United States saw the number of testosterone prescriptions triple, according to a 2013 study published in JAMA Internal Medicine. The researchers noted a corresponding trend of increased direct-to-consumer marketing leading men to believe "Low T" may be the underlying cause for their decreased sexual function and low energy.