Testosterone Not the Whole Story in 'Male Menopause'
Estrogen also involved, researchers say, and 'low-T' diagnosis isn't clear-cut
And that's a major issue, both Amory and Finkelstein said. Testosterone deficiency is diagnosed in millions of men each year, and in the United States, prescriptions for testosterone shot up 500 percent between 1993 and 2000 alone.
Right now, the definition of "low" varies, depending on the lab doing the testing, according to Amory.
But on average, testosterone levels below 300 nanograms per deciliter (ng/dL) are considered low, Finkelstein said -- and it's a sort of one-size-fits-all definition. "One of the goals of this study was to do better than that," he said.
To dig deeper, his team recruited 400 healthy men between the ages of 20 and 50. All of them took a drug that suppresses natural testosterone and estrogen production. Then, half of them used different doses of testosterone gel or an inactive placebo gel for 16 weeks. The other half used those gels plus a medication that blocked their testosterone from being converted into estrogen.
That way, the researchers were able to pinpoint the levels at which testosterone loss spurred particular symptoms -- and whether estrogen played a key role in any of those problems.
In general, they found, increases in body fat and a waning interest in sex started to emerge at mild levels of testosterone deficiency. But muscle loss and erectile dysfunction were not seen until testosterone dropped lower -- around 200 ng/dL or below.
Finkelstein agreed that none of this answers the big question of whether testosterone replacement is a good thing. And the researchers still need to see whether their findings hold true in men older than 50 as well.
"This study is a step along the way in our understanding," Finkelstein said. "I would tell men to stay cautious and conservative" about testosterone therapy.
Amory said there are some men who benefit from testosterone therapy, which comes in the form of topical gels, patches and injections, and can cost up to $300 a month.
But they are the men whose testosterone levels are clearly low and causing problems. The difficulty, according to Amory, is when a man has a moderately low "T level" and a vague symptom such as fatigue -- which can have many causes.