Of the dozens of known hormones in our body, perhaps none draws more attention—and controversy—than testosterone.

For some in the Trump administration, low testosterone (T) among teenagers is an “existential problem.” The Pentagon plans to test U.S. soldiers’ T levels to “optimize” their performance, even as the government tries to limit transgender people’s access to hormone therapy. On social media, meanwhile, men and women are promoting “T maxxing” for chiseled jawlines, bigger muscles and more energy.

The obsession is rife. In the July results of a poll by Men’s Health magazine, some 70 percent of men said their testosterone levels were important to them; almost half said these levels were a measure of “masculinity.” And a growing number of cisgender men are turning to testosterone-boosting supplements and synthetic testosterone, often delivered via injections, gels or patches, in a bid to up their levels. (This article primarily deals with testosterone therapy for cisgender men and women; you can read more about gender-affirming care for trans people here and here.)

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But does testosterone even carry any of these supposed health benefits? We asked the experts and dug into the science to find out.

What is testosterone?

Testosterone is a sex hormone present in people of all genders.

In men’s bodies, the hormone is primarily produced by Leydig cells in the testicles. These cells respond to signals from the pituitary gland to make testosterone. About 5 percent of testosterone is produced by the adrenal glands, which sit above the kidneys.

Testosterone is “responsible for a whole wide variety of functions,” says Michael Irwig, an associate professor of medicine at Harvard Medical School.

These include regulating the growth of hair, muscles and male genitalia, as well as supporting sperm production and helping drive libido.

Testosterone is also present in women’s bodies but typically at much lower amounts than are found in men. Women also produce testosterone via the adrenal glands, as well as from the ovaries.

In women, testosterone plays a role in bone health, fertility and sex drive, as well as menstrual health.

Do you need more testosterone?

“There’s this narrative that, if you have higher testosterone levels, you’re healthier, and you’re going to live longer,” Irwig says. “But there actually isn’t any evidence showing that.”

Importantly, for people diagnosed with testosterone deficiency, testosterone replacement therapy (TRT) can be an effective treatment. But as Scientific American has previously reported, taking testosterone comes with potential risks, including a decline in fertility, smaller testicles, acne, and more.

In adults, testosterone levels can be affected by several diseases, such as testicular cancer, as well as hypothalamic disease and pituitary disease, says Peter Snyder, a professor of medicine at the University of Pennsylvania.

The hypothalamus and the pituitary gland, which sit at the base of the brain, are like links in a chain in the production of testosterone; disrupting any of the links can throw off the body’s ability to make the sex hormone.

Damage to the testicles, such as from chemotherapy or surgery, as well as conditions such as HIV/AIDS, a mumps infection and obesity, can also affect testosterone levels, Irwig says.

Adults with severe hormone deficiency may experience symptoms such as a decrease in energy, lower sexual interest, reduced muscle mass and bone density and lower red blood cell production—all of which can be treated with testosterone therapy, Snyder says.

Currently, TRT is approved by the U.S. Food and Drug Administration for men with low testosterone “in conjunction with an associated medical condition.”

What about people without this diagnosis?

A truth of aging is that as men get older, their blood testosterone levels decrease. That’s spurred interest in whether TRT could hold benefits for older men.

In the early 2000s Snyder and his colleagues conducted a series of influential clinical trials called the “TTrials.” These included nearly 800 men with lower-than-normal testosterone levels who were assigned TRT or a placebo for a year.

“The results were pretty clear,” Snyder says: Testosterone boosted participants’ sexual activity and sexual interest, slightly improved their mood, increased their walking distance and bone density and upped the production of their red blood cells. But it also had a worrying possible side effect—higher volumes of plaque on the insides of the arteries, a risk factor for heart disease.

In 2018, at the urging of top FDA officials, the pharmaceutical company AbbVie and other drugmakers conducted the TRAVERSE trial to investigate potential cardiovascular issues linked to testosterone, as well as prostate cancer and bone fractures. It followed more than 5,000 men for nearly three years.

The trial found similar benefits to those observed by Snyder’s team; these included heightened libido and a modest improvement to mood. Also, the trial found that testosterone didn’t increase the risk of serious cardiovascular events such as heart attack or stroke in men at a higher risk of both conditions. Testosterone therapy also didn’t appear to increase the risk of prostate cancer. But the researchers did observe an increase in irregular heartbeat, lung-artery-blocking blood clots and—surprisingly—bone fractures.

Based on the results of TRAVERSE and other trials, in December 2025 the FDA convened a panel to consider approving testosterone for broader use, including so-called idiopathic low testosterone—that is, low testosterone that occurs for an unknown reason and is not necessarily associated with a disease—and removing warnings about the risk of prostate cancer.

What is considered “low” testosterone?

To be considered for TRT, the Endocrine Society’s guidelines recommend a patient have “unequivocally low” levels of testosterone.

What is “unequivocally low?” There isn’t a magic number. But doctors often diagnose low testosterone as below 300 nanograms per deciliter (ng/dl) across multiple tests. Blood concentrations of testosterone fluctuate, so diagnosis often requires several blood tests, Snyder says.

But the lower a person’s testosterone level—say, under 200 ng/dl—“the greater the likelihood of a benefit,” Snyder and a colleague wrote in a 2025 article in the New England Journal of Medicine. If a person has just slightly below average testosterone levels, the benefits of TRT may not outweigh the risks.

Low testosterone without a medical issue is somewhat uncommon. In recruiting for the TTrials, for instance, Snyder and his team screened more than 50,000 men with low libido, energy or mobility for low testosterone, defined as below 275 ng/dl. Although some of these men were disqualified for a variety of reasons, just 14.7 percent of those tested had low enough T across two samples to qualify for the trial.

For women, the Endocrine Society, as well as other leading health authorities, states that testosterone therapy may offer some benefit for postmenopausal women with hypoactive sexual desire disorder (HSDD) or clinically diagnosed low libido. But there are not yet any FDA-approved testosterone treatments for women.

What about supplements?

As for supplements that claim to boost testosterone, researchers say the scientific ground is shaky at best.

“There isn’t much evidence for dietary supplements to boost testosterone levels,” Irwig says. He’s unaware of any well-designed scientific studies testing whether they work as marketed. And as with most supplements, any claims made on the bottle are not verified by the FDA—it's bodybuilder beware.