In males, testosterone is mainly produced in the testes, and regulated by the brain’s hypothalamus and pituitary gland. Men with blood tests consistently showing low testosterone levels, along with signs or symptoms such as a depressed mood, lack of energy, low bone density, erectile dysfunction or decreased sex drive meet the criteria for an endocrine condition called hypogonadism.
Different countries and professional organizations have slightly different cutoff points in diagnosing the condition, but generally a concentration below 250 or 300 nanograms per deciliter of blood is considered low, said Dr. Shalender Bhasin, an endocrinologist and professor of medicine at Harvard Medical School.
The difference between healthy and unhealthy levels is rarely clear cut, however. Men with testosterone levels above 300 can still have a deficiency, and a low testosterone level may not produce symptoms. “It’s all a continuum,” said Elizabeth Platz, professor of epidemiology at Johns Hopkins University’s Bloomberg School of Public Health, who has studied testosterone. Researchers have defined a typical healthy range, but they have yet to determine a specific number within it that is optimal for any given man, but there isn’t a specific number within it that men should achieve, she said.
Hypogonadism also has several underlying causes, but experts disagree about whether all should be treated with testosterone. The U.S. Food and Drug Administration has approved testosterone products for hypogonadism caused by conditions that affects the testes, pituitary gland or hypothalamus, which affects less than one percent of men, said Dr. Bradley Anawalt, a reproductive endocrinologist at the University of Washington School of Medicine. But many men have low testosterone for others reasons: a high body mass index; a sedentary lifestyle; certain drugs; and conditions such as sleep apnea, diabetes and anemia. Levels also tend to decline as men age, with roughly 10 percent of men in their sixties classified as hypogonadal.
As testosterone declines, some men may experience a low libido, depressed mood or decreased energy even if their levels are not technically low, Dr. Anawalt said, adding that some doctors will not prescribe testosterone to these men, but others might.
A drop in testosterone from the high end of normal to the lower end doesn’t always indicate a problem, though. Testosterone levels fluctuate from day to day, and even within a single day. Some tests are also notoriously inaccurate, Dr. Anawalt said.