Odds are, you’ve heard of “Low T.” That’s the pharmaceutical industry’s name for testosterone deficiency in men.

There’s a lot of attention given to treating it, but far less emphasis on a national health crisis: mysteriously declining testosterone levels in American men population-wide. They’ve been losing an alarming amount of the crucial hormone that quite literally makes them men. And nobody has identified exactly why.

“Testosterone peaks in the late teen years, in the early 20s,” said Abraham Morgentaler, a Harvard testosterone researcher.

Morgentaler says he began seeing patients with testosterone deficiency in the late 1980s. The men were desperate for help for sexual dysfunction. He treated them with testosterone replacements and has consulted for a company that makes a therapy.

Morgentaler says there are different ideas about what exactly is too low and what treatment, if any, men should get. But scientists say there’s no doubt that male hormone levels have shrunk in recent decades. And we’re not talking about common age-related decline. Men in their 40s today, on average, have significantly less of the male sex hormone than their fathers did when they were 40.

“It depends on the study, and there have been several, but it ranges anywhere from 10 to about 20%… over the last 40 to 50 years,” Morgentaler said.

He added, “So today, the average testosterone level is about 450 nanograms per deciliter, we say. There’s a lot of variation from one person to another, but that’s an average of middle-aged, relatively healthy men. So what was it 40, 50 years ago? It was probably around 500 or 550.”

On top of that, it’s estimated that many millions of men have testosterone levels well below the current, lower average. The problem is reported elsewhere but is said to be much worse in the U.S. than in other parts of the world.

“It’s hugely prevalent, affects about one out of three adult men over the age of 40. And not only does it cause symptoms that make the men not feel so good, but it’s associated with important health issues.”

When testosterone is low, it means the testicles aren’t producing an adequate amount. That can lead to all kinds of signs and symptoms, including lower sex drive, erectile dysfunction, fatigue, brain fog, loss of muscle mass and strength, more body fat, weight gain, breast growth, sleep problems, mood swings, weak bones and fertility problems.

“The doomsday scenario, if things continue the way they are, is not only that testosterone levels decline, but also sperm numbers,” Morgentaler said. “And there’s evidence that sperm numbers are declining too. They’re both made in the testicle.”

He added, “We could have sexual problems that are more prevalent. And sex is actually about reproduction, so that if men can’t perform sexually to a certain extent, that may affect birth rates, which are already going down in the western, developed world anyway. I think it’s possible — but I hope we never get there — that fertility will be reduced, that men will have more and more difficulty being sexual and active and physical, and that men will start feeling old before their time. And society will feel the impact of all of that.”

The same exposures disrupting hormones in men could be disrupting the hormonal balance in women. But there seems to be no major public effort to get to the bottom of that either.

“Full Measure with Sharyl Attkisson” airs at 10 a.m. Sunday, WJLA (Channel 7) and WBFF (Channel 45).