A new plan to conduct testosterone screenings for all U.S. troops 30 years and older has raised many questions among the general population.

Defense Secretary Pete Hegseth is calling for mandatory annual tests to see if men are deficient in the sex hormone and if they want to receive testosterone therapy.

No major medical society recommends routine testosterone screening of healthy men. Just about everything about testosterone is controversial, from how to define a deficiency, to what level supplementation should reach, to the potential risks of therapy.

Here's what we know.

What is testosterone and what's a normal level?

Testosterone is the main male sex hormone, though women, too, have testosterone at lower levels.

The normal distribution of testosterone levels in men is wide-ranging, from a low of 264 nanograms per deciliter of blood to 914, says Dr. Shalender Bhasin, a professor of medicine at Harvard Medical School who has studied testosterone for 40 years. Testosterone is released in bursts roughly every 90 to 120 minutes, says Bhasin.

So levels can vary by as much as 15% to 20% during one day, says Dr. Brad Anawalt, an endocrinologist and professor of medicine at the University of Washington Medical Center in Seattle. That's why levels should be tested when they peak, which is in the morning between 7 a.m. and 10 a.m.

Testosterone levels can be lower from illness, significant trauma or even chronic poor sleep from conditions such as sleep apnea. "It's always important to measure these testosterone concentrations when you're in your usual state of good health," says Anawalt.

Do testosterone levels decline with age?

Not as much as many think.

Testosterone levels typically peak in your 20s or 30s and then decline by about 1% a year. Men who are healthy and lean typically don't have these drops.

"Most of the effects of aging are related to weight gain and obesity," says Anawalt.

Testosterone levels become affected when men gain more than 5% of their body weight or when their body-mass index $(BMI)$ starts creeping up past 25 and certainly over 30, he says.

For a small percentage of men over age 70, testosterone levels may drop below the normal lower limit due to aging, says Anawalt.

Besides obesity, what other factors can contribute to testosterone deficiency?

Chronic diseases related to obesity such as Type 2 diabetes can also result in lower testosterone levels.

Many medications can also lower levels, including opioids, anabolic steroids and glucosteroids, used to treat many inflammatory conditions, as well as treatments such as chemotherapy. Health conditions like pituitary-gland issues and genetic disorders such as Klinefelter syndrome can also result in testosterone deficiency.

Physiological stress, such as consistent sleep deprivation from sleep apnea, not eating enough calories, or excessive exercise can result in a low testosterone.

What are the risks of being testosterone deficient?

In addition to a lower sex drive, having testosterone deficiency can lead to decreased bone density and muscle mass, as well as a decrease in the amount of red blood cells you produce.

Roughly 2% to 7% of men over the age of 35 have testosterone deficiency. The majority of those men are obese; fewer than 1% have an underlying disease or medical condition that results in lower than normal testosterone levels.

How to measure testosterone levels, and should you seek therapy?

Levels are measured by a simple blood test. But they aren't typically checked in annual wellness exams unless requested. And no professional doctor's association recommends widespread screening in the general population.

Is therapy a must for low levels? Not necessarily. Doctors are clear that treatment is only needed if your levels are low and you experience symptoms such as low libido, low energy and mood, and loss of muscle mass or bone.

Bhasin says certain at-risk subsets of the population should be screened, such as people with chronic kidney disease on dialysis, men who have been treated with chemotherapy, or have chronic liver disease. Others include men with sexual dysfunction, infertility or breast enlargement.

What is testosterone therapy and what are the risks?

Testosterone therapy can be delivered through the application of a daily gel, weekly home injections, twice-a-day pills or an implanted pellet.

The main risk is the potential for infertility due to a lower sperm count, says Dr. Adrian Sandra Dobs, professor of medicine and oncology at the Johns Hopkins University School of Medicine. Once a man stops the therapy, it can take months for the sperm count to be restored.

"Do you really want to start giving testosterone to young men in the peak of their reproductive years which might result in a drop in sperm count and their own production of testosterone?" she says.

Another potential side effect is an increase in the number of red blood cells. This can make the blood thicker and raise the risk of blood clots, strokes and heart attacks.

So is testosterone deficiency overdiagnosed or underdiagnosed?

Depends on how you define it and whom you ask.

Even though the American Urological Association guidelines don't recommend annual screenings, they support the administration's move to screen troops for testosterone deficiency, noting that having a baseline value even in the absence of symptoms can be helpful. The Endocrine Society, on the other hand, doesn't advocate screenings for all troops.

Anawalt says screening large populations of adult men for testosterone levels will result in a lot of false positives. Therefore, it's best to only screen men with symptoms.

But Dr. Helen Bernie, an associate professor of urology at Indiana University and a spokesperson for the AUA, says testosterone deficiency is often underdiagnosed. Even if a man doesn't have symptoms, just knowing they have low testosterone can be a red flag since it's associated with higher rates of obesity and other chronic health conditions.

"Low testosterone in itself can be an early biomarker for man's overall health," says Bernie. "This could be a warning sign of other underlying conditions that we can actually treat."