NEW YORK - Low testosterone production can reduce strength, muscle size and lean mass. If the testosterone isn't being converted to estrogen, men may see an increase in body fat. And a deficiency of either can spark a decline in sexual function, according to a new study of hundreds of volunteers.
The research, published in the New England Journal of Medicine, comes at a time when testosterone supplements are being widely advertised to men in the US, for example, in commercials urging them to be treated for "Low T."
From 2001 to 2011, the number of US men receiving such treatment more than tripled and replacement therapy has become a nearly-$2 billion business.
Unlike new drugs that are approved for a specific problem, testosterone is subject to "surging overuse from off-label prescribing for diverse unproven indications, including use in older men as an anti-aging or sexual tonic and in younger men for bodybuilding or doping," even though the way it works is not well understood, David Handelsman of the University of Sydney writes in a Journal editorial.
"The bottom line is there are tens of millions who are aging and who are having their hormone levels changed," lead study author Dr. Joel Finkelstein of Massachusetts General Hospital in Boston told Reuters Health. "We don't know which men should be treated, whether it's effective and whether it's actually safe."
The new study was an attempt to separate the effects of testosterone and estrogen - most of which is made by the body from testosterone - on men's sexual function, strength, muscle size and lean and fat mass, and to discover how much testosterone is needed for health.
"This study allows us to begin to be more sensible about the diagnosis and treatment of low testosterone," Dr. Bradley Anawalt, chairman of The Endocrine Society's Hormone Health Network, told Reuters Health. He was not connected with the research.
"It shows that testosterone has different effects at different levels for different tissues," he said.
A group of 400 healthy volunteers were injected with a drug to shut down natural production of both testosterone and estrogen, then for 16 weeks they were given varying doses of testosterone gel, placebo gel and anastrozole to prevent testosterone from being converted to estradiol, a form of estrogen.
Among the findings, sexual desire dropped progressively with lower testosterone levels.
Testosterone levels had to be extremely low, however, before erectile dysfunction appeared. But if estrogen production declined, that had an effect on sexual function regardless of how much testosterone the man was producing.
Testosterone levels also had to be very low before the researchers saw declines in leg strength, the size of the thigh muscle and lean body mass.
When estrogen production was blocked, there was an increase in body fat regardless of how much testosterone was in the blood. When it was not blocked, only a mild reduction in testosterone was enough to increase body fat.
But estrogen levels alone had no effect on leg strength, muscle size or the leanness of the body.
The researchers said that, for the moment, symptoms - and not just a man's testosterone level - should determine whether he is a candidate for testosterone replacement therapy.
The pattern of results also shows that if testosterone therapy is given, it should be in a form that the body can convert into estrogen, they said.
The men in the study were ages 20 to 50 years old. The researchers hope to run the same test on older men to see if the results hold for them.
"The real big picture here is what happens with aging," Finkelstein said. "This is a huge public health issue. At middle age, testosterone levels do decline modestly. At the same time, men have other changes that can be associated with testosterone. They lose muscle mass and gain fat, and lose sex drive and lose bone, etc. The question is whether the changes that occur with aging are causally related to the changes that occur in hormones."
"Longer studies are required to partition the effects of testosterone on bone density and fractures, or on prostate growth and diseases," Handelsman said in his editorial.
Similarly, he pointed out, an "equally demanding analysis" could shed light on testosterone's influence over metabolism, cardiovascular heath, memory, mood and behaviour, as well as the effects of age and obesity.
In this study, the men started out with testosterone levels that averaged more than 500 nanograms per deciliter of blood. Fat accumulation began when the level dropped to about 300 to 350 ng. Lean mass, muscle strength and thigh-muscle area started to drop at 200 ng.
"I think this will help clinicians make more rational decisions about whom to treat with testosterone," said Finkelstein. "It also teaches clinicians that there is no one answer to what's a low level. If the patient's interested in sexual function, it's one level. If they're interested in bone loss, it's another level. Unfortunately it's not one size fits all."
Anawalt, of the University of Washington in Seattle, said he is concerned that some people will use the new findings to push for more aggressive testosterone treatment. He said that would be irresponsible.
"In practice, a lot of clinicians who are prudent and cautious have said, 'Maybe the patient's symptoms are due to low testosterone.' But generally, the results are pretty disappointing when you give it," he said.
In his practice, "If they come in with a little extra weight and their testosterone levels are on the low end of normal, I tell them you have to diet, start exercising four to five times a week and usually your testosterone levels will start coming up," said Anawalt. "That's the right answer for a lot of these guys."