What does testosterone do? More than most men are told. It holds your muscle and bone, sharpens your mood and focus, steadies your metabolism, supports your heart, and drives your libido and energy.
Why does it fall? Two clocks run at once: age, which takes about 1 percent a year after 30, and your generation, because men as a group have drifted lower over the decades. A man in his forties often starts below where his father did at the same age. Much of the fall tracks with sleep, weight, and habits, so you can change a good part of it.
The man who was told he was fine
I see this man often. He is in his late 30s or early 40s, and he tells me he doesn't feel like himself. He is gaining a little weight around his midsection and having a hard time building muscle.
He doesn't have the libido he used to, he doesn't have the energy, and he doesn't have the motivation to go to the gym anymore, and he feels lackluster.
He went to his primary care provider first and had his testosterone read. It came back in the normal range, and his provider said: You're fine, this is normal.
Then he sits down with me, and his full panel shows what the first test could not. His total testosterone reads inside the normal range while his free testosterone sits low. He has a biologically low amount of testosterone available for his body to use.
What changed it for him was the whole picture. He learned how testosterone touches so many parts of his life, what an optimal level is, and how the standard range keeps shrinking. Let's start with the first part.
What testosterone does across your body
Testosterone works in nearly every system you care about. In muscle, it signals tissue to build and hold protein, so it sets how much lean mass you keep and how well you recover from hard training. In bone, it feeds the cycle that keeps your skeleton dense.
Your brain reads it too. Receptors for testosterone sit in the regions that govern mood, motivation, and focus. So a falling level often shows up first as flat mood and low drive, before anything physical changes.
Metabolism is the quieter work. Testosterone helps regulate how your body handles fat and insulin, so a lower level tends to mean more weight around the middle and a harder time holding muscle. Your heart and blood vessels sit inside that same metabolic picture.
Then come the parts men notice first: libido and energy. They tend to move together when a level shifts.
Because testosterone works everywhere, I read it as part of hormones as one system, alongside thyroid and the rest. One hormone, working in every system, tells you little as a lone number.
How testosterone falls with age
Testosterone peaks in your early twenties, then declines at about 1 percent a year after age 30. In any one year the drop is small enough to miss. Across three or four decades it moves you a long way from where you started.
The total number tells only part of the story. Most of the testosterone in your blood rides bound to a carrier protein called SHBG, and the bound part is not available to your tissues. As you age, SHBG rises, so your free testosterone falls faster than your total does.
A man can hold a normal total while the free testosterone his body can use has quietly dropped below what he needs.
Remember the man who was told he was fine? This is the number his first test never measured.
And the range he was measured against keeps moving. In the years I have been treating men, the top of the standard reference range has come down from 1500 to 1000 nanograms per deciliter. The range keeps getting smaller, so a "normal" result today clears a lower bar than it used to.
Why you often sit lower than your father did
Men as a group have been drifting lower for decades, on a clock separate from age. Researchers who followed American men for twenty years found a population-level decline in testosterone that age could not explain. A 60-year-old measured in the early 2000s carried less than a 60-year-old measured in the late 1980s, and the same held across ages.
I see the pattern in my own patients. I often have men in their 70s with higher testosterone than a man who is 35, and a lot of that comes down to the way they grew up and the type of work they have done. When I look closer at the younger man, it is his stress, his diet, his activity, and his sleep.
Sometimes he is training hard and still sitting low, because he is overtraining and not sleeping enough, so he is suppressing his own production.
And some of it is what surrounds him. The types of food he eats, the things he cooks in, and what he is exposed to all press on the same system. There are a lot of endocrine disruptors now, and we are exposed to them daily.
Why the whole population drifted is not settled, and the study could not pin it to weight or smoking. Whatever the cause, the starting line has moved, so a level that is common for your age can still sit far from optimal for your body.
How much of the fall you can change
More than you might think. When researchers accounted for conditions that ride alongside aging, like obesity, diabetes, and heart disease, age on its own predicted much less of the decline. A good share of what looks like an age effect is a metabolic one, carried by weight, blood sugar, sleep, and movement.
So the same number can sit differently in different men. A total of 400 nanograms per deciliter reads one way in a lean, well-slept 35-year-old and another way in a 55-year-old carrying extra weight and broken sleep.
Sometimes I have to be pretty straightforward about this with a patient. I will say: Your inactivity, unhealthy diet, and lifestyle habits have created this hormone suppression, and here are the ways you can fix it, if you're willing to.
The fix depends on your age, honestly. If you are in your late 30s with a long list of factors you can change, I do not like to start you on testosterone. I ask you to change the diet part, the sleep, and the exercise first, and I often see a huge change in a man's own testosterone production.
I use a couple of supplements alongside those changes. One I reach for is LibidoStim-M by Designs for Health, a blend of nutrients and botanicals for male hormone health.
When we cannot get a younger man's level as high as I would like, I use enclomiphene instead of straight testosterone therapy. Enclomiphene stimulates your body's own production, and that often works great as well.
A low number is never the whole panel for me, either. I also look at IGF-1, one of your growth signals, along with your thyroid and your nutrients, especially B12 and vitamin D. A lot of men run low on both, and you need them to make hormones. Bringing them up can change your treatment and how you feel once you are optimized.
What your number is telling you
So what does testosterone do, and why does it fall? It runs muscle, bone, mood, focus, metabolism, and drive, and it falls on two clocks at once: the slow pull of age, and a load of weight, sleep, stress, and exposures that you carry with you.
Nobody escapes the first clock. The second one is yours to work on, and it is often the bigger of the two.
So the better question to bring me is where your free testosterone stands, and how much of the gap is yours to close.
That answer sits in your own numbers. And most men are surprised by how much of it they can move.