Drooping testosterone levels probably don't cut years off a man's life, although earlier studies had suggested they might, according to a new report.
Instead, decreases in the male sex hormone may simply be a sign of overall health status, which also dips with age, researchers say.
The findings come in the wake of surging interest in testosterone's role in men's sex drive, their mood and thinking, and even early death.
While drug companies refer to "low T" as a treatable medical problem affecting millions of American men, critics say they are trying to make a buck by turning normal aging into a disease.
There is no doubt that very low testosterone levels affect the body and dampen sex drive.
But where to set the threshold between normal and abnormal levels is still an open question, whose resolution is not made easier by natural variation in the male sex hormone.
It dwindles with age, for instance, and fluctuates between countries, races and individuals -- even from one hour to the next.
The new study, published in the Journal of Clinical Endocrinology & Metabolism, takes an in-depth look at earlier research into the link between testosterone and death.
"It's the ultimate endpoint," said Andre Araujo of the New England Research Institutes, who worked on the study.
So far, he said, most but not all research shows men with low testosterone die before their peers with higher hormone levels.
"The results were disparate, they were confusing," he told Reuters Health.
So he and his colleagues gathered all the medical literature they could find on the topic, including a report of their own that had found no link.
They only chose studies of men living in the wider community, as opposed to hospitals or long-term care, to try to avoid the muddying influence of severe illnesses on the data.
CORRELATION, NOT CAUSATION
Based on 11 studies, which tracked more than 16,000 middle-aged men for between five and 17 years, the researchers did find signs of trouble: it appeared that men in the lowest third of testosterone levels had a 35-percent higher chance of dying than men in the highest third.
"There is certainly a link," said Araujo. "But the important thing is that the estimates are a little bit off from one another."
That means they aren't really comparable, statistically speaking.
Digging deeper, Araujo found that shorter studies were more likely to find a connection, as were those including mainly older men.
On the other hand, low testosterone levels did not seem to be associated with early deaths in younger men.
Although the work is limited by including only 11 studies, "these data indicate to me that testosterone seems to travel with declines in health and (with) mortality, but it is not really driving the mortality," Araujo said.
The new results add to earlier evidence that companies promoting testosterone replacement for "low T" may be jumping to conclusions, or at least reaching beyond the evidence.
Experts say more than a million testosterone prescriptions are written in the U.S. every year, and many go to middle-aged and older men with stunted libido and depressed mood.
A Google search for the phrase "low testosterone," for instance, yields a link to the "Is It Low T" website, owned by Abbott, which produces a testosterone replacement product called AndroGel that costs about $280 per month.
On the site, a visitor can take a quiz asking about things like grumpiness, lack of energy and decreased libido.
If you answer yes to three questions, or have decreased libido or "less strong" erections, you are urged to "Talk to your doctor about your symptoms and ask if you should be tested for low testosterone (Low T)."
But so far, there isn't any solid research to prove that dips in testosterone will necessarily cause those problems -- or that testosterone therapy will fix them.
"Causality, I mean hey, causality is a very tough bar to reach," said Araujo.
He is a consultant to drugmaker Eli Lilly, which makes a testosterone gel called Axiron, but declined to spell out what he does for the company.
Araujo said the National Institutes of Health is currently running a trial to test the efficacy of testosterone replacement therapy.
Last year, Dr. Michael Marberger, who heads the urology department at the University of Vienna Medical School in Austria, told Reuters Health testosterone replacement has become a common fix for sexual problems.
"It is like putting more gas in the car to make it go faster," he said, adding, "We need a lot less testosterone for sexual function than people used to think,"
Abbott did not respond to an email request for comment.
According to the company, nearly 14 million American men "may have" low testosterone. But independent work suggests the number of men who qualify for a strict diagnosis of "late-onset hypogonadism", in which the testicles stop or almost stop making testosterone, is much lower.
SICK OR JUST GETTING OLD?
Meanwhile, sociologists are worrying about the effects of putting a medical label on what many consider normal aging.
"With something like sexuality and sexual performance the medicalization has gone pretty far down the road," Susan Bell, a social scientist at Bowdoin College in Brunswick, Maine, told Reuters Health.
"Essentially it says, you should have the same kind of sexual function as you did when you were 20 or 30. If you don't, there is something wrong with you, and therefore you can use Viagra or another drug."
She said the push now for men to use testosterone to stave off aging mirrors the earlier urging of women to start hormone replacement therapy around the time they hit menopause to preserve youthfulness.
Yet in effect, that treatment turned out to do more harm than good for many women, producing slight increases in heart attacks and breast cancer among other problems.
"The cautionary tale is that hormone replacement has had damaging effects on women's bodies that have become apparent over time," she said, adding that some women with severe menopause symptoms might still benefit.
"One of the messages for men is, what does this mean for the rest of your life and what do we know about dangers of taking a medication over a long period of time?"
Short-term side effects of taking testosterone are swelling around the joints and in the breasts, but the long-term consequences are less clear.
Bell said the new study strikes a cautious note that is less commonly heard in industry marketing messages.
"That kind of gap between the uncertainly in the medical community and the enthusiasm in the pharmaceutical industry is similar to what happened with women's hormone replacement therapy," she said.