When 33-year-old Ryan Hall told the running world of his retirement last week, he cited chronically low testosterone A Part of Hearst Digital Media.

Best known for building bodybuilders’ biceps, the sex hormone plays a crucial role in endurance sports as well, and training can affect levels in some surprising ways. We spoke with two experts about how running and testosterone intertwine—and why treatment isn’t always straightforward.

What does testosterone do for you, exactly?

As the main sex hormone for males, testosterone fuels facial- and body-hair growth, sex drive, and sperm production. Testosterone plays a key role in building muscle, helping your body produce and assemble the proteins required to construct lean mass.

Though most runners wouldn’t want a Schwarzenegger-like physique, a dropoff in muscle strength can lead to fatigue that impairs performance in endurance sports like distance running, says endocrinologist John Morley, M.D., a low testosterone expert at St. Louis University School of Medicine.

Testosterone also protects men’s bone health, much the way estrogen works for women (who, though they have some testosterone, too, don’t need nearly as much—and essentially no research links low levels to poor performance in women).

Finally—critical for runners—testosterone boosts red blood cell count, says University of Washington endocrinologist Bradley Anawalt, M.D., a spokesperson for the Endocrine Society. The more red blood cells you have, the more oxygen they can carry to your working muscles. The better this process works, the longer you can push the pace.

Specific symptoms of low testosterone in men include a drop in sex drive, a low sperm count, enlarged breasts, and fewer early-morning erections. Over time, men can develop other, vague signs—including fatigue, a low mood, loss of strength, and weak bones.

So what’s going on when you have low levels?

Testosterone levels naturally decline with age, beginning at around 30. In younger men, low testosterone can occur when one or both testicles sustain damage, as a side effect of opioids or other medications, or due to a genetic defect.

Another common culprit: a tumor in the pituitary gland, a tiny organ near the base of the brain that releases a compound called luteinizing hormone. Leutenizing hormone gives the testicles their testosterone-producing orders. Without it, the sex-hormone assembly line slows or stalls.

In normal situations, endurance training may cause slight declines in testosterone levels, but usually not enough to cause problems, Morley says. Rather, problematically low T comes as a consequence of overtraining. Placing your body under more stress than it can handle sends levels of a hormone called cortisol skyrocketing, which may throw a wrench in your testosterone production.

Scientists suspect dangerously low body fat percentages that often occur with overtraining also play a role, Anawalt says. Fat cells crank out a hormone called leptin, which signals to your brain that you have enough food to avoid starvation. Without the all-clear, your brain shuts down sex hormone production. After all, if you’re scrambling to eat, you probably shouldn’t be trying to reproduce, Anawalt says: “That would be a waste of time and energy and you would be a poor parent at a time when you should be conserving all of your energy for survival.”

Other signs of overtraining include trouble sleeping and the same types of fatigue and performance declines that can occur as a result of low testosterone, making it difficult to untangle cause from effect. “When you suddenly find that you’re doing all this training and you’re slowing down, not going faster, it’s a fairly good sign that you’ve overtrained,” Morley says.

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Though it might seem black and white—you have enough testosterone, or you don’t—hormones rarely play by simple rules. For one thing, testosterone levels vary by individual and also fluctuate naturally, declining throughout the day, Morley says. And a number that triggers one man’s extreme fatigue may leave another feeling totally fine.

To even undergo testing for low T, you should first have symptoms such as fatigue and sexual complaints, Anawalt says. For an official diagnosis, you’ll need two morning blood tests a week apart, both showing low readings (typically 300 ng/dL or lower, depending on the lab your doctor uses).

Besides the immediate decline in energy, performance, and sex drive, low testosterone has damaging effects over time. Especially for athletes in their prime reproductive years, extended periods without sufficient sex hormones can permanently impair fertility, Anawalt says.

Plus, piling miles on top of bones weakened by months of low testosterone levels can add up to osteoporosis and stress fractures. For years, doctors have warned young female runners about a constellation of poor nutrition, stopped periods, and low bone mass called the female athlete triad—now, they believe young men can develop a similar problematic pattern.

So, what do you do about it?

If your doctor determines you have low levels, he or she will try to figure out why, a process that involves looking for tumors, examining your testes, scoping your prescription list—and, for athletes, asking about your training habits.

For athletes with overtraining-induced low testosterone, the simplest and most effective treatment is to back off a bit, Anawalt says. Often, a few weeks or months off—or at least with reduced mileage—will bring levels back up to normal. Eating a few more calories and regaining body fat helps, too. “The turnover if you’re not overtraining is very quick; testosterone just replaces itself,” Morley says. “There is no evidence I’ve seen that says that these people are going to be down forever and never recover.”

Hall has credited weight training with improving his levels. Strength- and power-based activities do cause a short-term boost in testosterone levels, Anawalt says. But that’s not to say runners have to pump serious iron to stay healthy. Studies haven’t yet shown that these spikes translate into sustained increases over time. Cutting back on overtraining, and the accompanying increase in body fat, play a much larger role in restoring hormonal balance, he says.

Then what’s the deal with testosterone therapy?

For people with low testosterone caused by such factors as tumors or genetic defects, testosterone therapy—administered through injections, smeared on in a gel, or absorbed through a patch on your skin or a tablet on your gums—can restore sex drive, improve erections, boost energy and mood, and rebuild muscle and bone strength.

However, younger athletes have reasons to steer clear. For one thing, adding testosterone from the outside can reduce sperm production over time, making it much more difficult to have children later on, Morley says. Other side effects include an increased risk of prostate cancer and potential for heart attacks and other cardiovascular issues as well—though the links aren’t totally clear, and pose more of a problem for older men.

Finally, there are doping regulations. By boosting red blood cell count and reducing recovery time—a poorly understood effect that could be due to either physiological or psychological factors—testosterone confers an unfair advantage to athletes. That’s why it appears on the banned substance list for both the World Anti-Doping Agency and its U.S. counterpart.

And though athletes with a genetic defect or pituitary tumor qualify for what’s called a therapeutic use exemption, Anawalt (who consults for USADA) says the same doesn’t apply to cases caused by overtraining. “Of course, we don’t have access to their training records, but what we have access to is their body mass index and their weight,” he says. If those numbers suggest an abnormally low body fat and the athlete provides no evidence of another cause for low testosterone, he’ll deny the application. “What I write in all my denials—I say, ‘This doesn’t mean that a sensible doctor out there might not prescribe testosterone, but they don’t qualify for a therapeutic use exemption.’”

Headshot of Cindy Kuzma
Cindy Kuzma
Contributing Writer

Cindy is a freelance health and fitness writer, author, and podcaster who’s contributed regularly to Runner’s World since 2013. She’s the coauthor of both Breakthrough Women’s Running: Dream Big and Train Smart and Rebound: Train Your Mind to Bounce Back Stronger from Sports Injuries, a book about the psychology of sports injury from Bloomsbury Sport. Cindy specializes in covering injury prevention and recovery, everyday athletes accomplishing extraordinary things, and the active community in her beloved Chicago, where winter forges deep bonds between those brave enough to train through it.