Low testosterone, or hypogonadism, is a legitimate health problem. Men who have it can suffer various conditions, from low energy and sex drive to depression, weight gain and even bone loss.

But chances are you don't have this problem, and neither do many of the men you know.

The best evidence is that only about 2 percent of the U.S. male population ages 40 to 79 are below the minimum T threshold.

That hasn't prevented a wave of interest in testosterone-replacement therapy. At the top of the hierarchy are prescription-only gels and injections that replace the diminished hormone with synthetic testosterone. At the other, much more dubious end of the spectrum are the dozens of over-the-counter supplements that purport to help the body boost its own T levels naturally.

"It's just a bunch of nonsense," Dr. Ellis Levin said of the supplements. He's chief of endocrinology, diabetes and metabolism at the School of Medicine at the University of California, Irvine. He said those products aren't FDA-approved, nor are they generally tested to see how well they work. "So people can claim whatever they want, and nobody will hold them to the truth."

Often, when a user does find a benefit, it's a placebo effect that dissipates over time, he said.

The supplements, with names such as Manimal, HexaTest and High T, often make bold claims in advertising and marketing. Andro400 says on its website that its capsules are "powered by testosterone," which can have various therapeutic properties, including the ability to "melt" body fat. Elsewhere on the exclamation-point-filled home page are slogans such as: "Get back your enthusiasm, motivation and zest for life!" and "Enhance sexual performance and bring back romance!" One 60-pill bottle costs $39.95.

Vanishing testosterone

Is it true that men produce less testosterone as they age? Generally, yes. But that might not mean much. And there are other factors that can bring the level of hormone down, including injury, illness, obesity and heavy use of marijuana and opiates.

A 2012 study found that men who were married tended to have less precipitous drops, possibly owing to being happier or having more sex. However, a 2011 study said new dads can see their levels drop faster than non-dads, an effect possibly caused by the body's need to nurture a child outweighing the need to perform historically macho tasks, such as fighting a rival suitor.

Of course, many of the symptoms associated with low testosterone, such as fatigue, could come from other sources. For instance, erectile dysfunction is more often caused by cardiovascular disease than by low T.

A normal level of testosterone is 300 to 1,100 nanograms per deciliter of blood; a test is normally given twice — in the morning, when levels are at their highest, and after fasting. A low reading means the testicles aren't producing enough testosterone, but most of the time the problem originates in the brain, Levin said: There's a mix-up in signals from the hypothalamus or the pituitary gland, which tell the testicles when to make more.

The best evidence is the tiny percentage of U.S. men who are below the minimum T threshold. But a study published in June in JAMA Internal Medicine showed that prescriptions for men 40 and older shot up 300 percent from 2001 to 2011. About one-quarter of them had not received a testosterone test at all.

Although a study in Germany last year chronicled consistent weight loss among study subjects who had testosterone by injection (not available in the United States), there's little research on the long-term effects of T replacement in men who don't have low levels.

Synthetics vs. supplements

The industry leader in synthetic testosterone is AndroGel, a name similar to Andro400 (androgens are male sex hormones, testosterone being the major one). The gel, developed by Solvay Pharmaceuticals and approved by the FDA in 2001, is rubbed directly on the skin. It comes in two concentrations, 1 percent and 1.62 percent, and children and women are cautioned against touching the areas that aren't clothed or washed off after the higher-concentration version is applied. The website also warns that using 1.62 could reduce a man's sperm count. The site encourages men with symptoms of hypogonadism to consult with their physicians and "ask to be evaluated," including possibly getting a blood test.

The supplement makers tend to be less explanatory about their products. Andro400 contains Eurycoma longifolia, a flowering plant native to Indonesia. Peter Jenkins, manager of the customer-support team for Andro400, says Eurycoma longifolia works by stimulating the hypothalamus and the pituitary, helping the body raise its own levels of testosterone.

He said synthetic testosterone carries "risks" and "negative side effects," one of which is that it could result in the body further reducing its own production of natural testosterone.

Dr. Levin said that claim is absurd.