Depending on age and individual makeup, typical testosterone levels in adult men range widely, from 280–1,100 ng/dl, according to researchers at the University of Rochester. Most adult men average about 679 ng/dl, although some researchers suggest that 400–600 is optimal in healthy individuals. Testosterone decreases naturally with age, but slowly, at a rate of about 1 percent a year after age 30. Low testosterone can also be caused by tumors in the pituitary gland or testes, especially in younger men, as well as diseases such as type 2 diabetes. It also has been strongly linked to obesity.
So what are the signs of low testosterone? They include low sex drive, fatigue, loss of muscle mass, decreased bone mass, increased body fat and mood changes, including irritability or lack of focus.
Multiple studies have found that testosterone effectively boosts libido and sexual function for older men with hypogonadism. TRT also has been linked to modest increases in bone density, muscle tone and grip strength.
But few of the other claims about TRT, which typically costs about $400 a month and is sometimes not covered by insurance, pan out. For instance, several recent studies corroborating TRT's positive effects on libido in older men with hypogonadism also found that TRT had no meaningful effect on fatigue, depression, memory, emotional state, enhanced cognition, mood or energy — the two most common reasons men give for seeking out the drug, other than libido — compared to control groups. In addition to the increased risk of heart disease and stroke, scientists currently are debating the role of TRT in the development of prostate cancer.
Comparatively, hypogonadism affects about 3 percent to 5 percent of men under age 45. For them, side effects may be much more dangerous, including permanent infertility, said Halis Kaan Akturk, a professor of medicine at the University of Colorado and formerly a doctor at the Mayo Clinic. Akturk said he sees at least one patient per day seeking testosterone, many of them former student athletes now in their 20s and 30s, who began taking testosterone early and are now dependent.
"If we give someone testosterone, we are making their testicles lazy," Akturk said. "In these patients, who have been using testosterone for five to six years, their testicles get atrophied and sometimes permanently damaged. It's a hard thing to wean people off of."
What's more, abnormally low testosterone in younger men can indicate a different, often serious problem, which artificially upping testosterone levels can make the both harder to detect and treat, Akturk said. Treatment for prostate cancer, for instance, a common cause of abnormally low testosterone in young men, typically involves decreasing the amount of testosterone in the bloodstream. Excess testosterone can "wake sleeping tumors," Akturk said.
Akturk was part of a team of Mayo Clinic doctors who treated a 56-year-old man who went blind in both eyes soon after starting testosterone therapy prescribed by his doctor. Akturk and the other doctors traced the blindness — as well as the man's original symptoms, for which he sought testosterone therapy and which still had not abated — to a tumor in his pituitary gland. The testosterone, they found, had stimulated the tumor further.
After stopping testosterone therapy and prescribing medication to decrease the amount of testosterone in the bloodstream, the man's eyesight mercifully returned. "It needs to be worked out why this person has low testosterone, rather than just giving them testosterone," Akturk said.
That does not appear to be happening in practice, however. Only half of the men at a medical center interested in TRT had a clinical diagnosis of hypogonadism, according to a 2017 study by researchers at Emory University School of Medicine, and about 10 percent knew about side effects.