In a 2025 study of 557 men, the free fraction of testosterone tracked muscle mass, grip strength, leg power and physical function in the men over 65. Total testosterone, the number you were almost certainly shown, largely did not.
You went in because something had changed. Strength that used to be there. Drive that used to arrive on its own. The same training, less to show for it. Blood was drawn, one number came back inside the reference range, and the conversation ended. You left with a normal result and a body that disagreed with it.
There is a specific, measurable reason both of those can be true at once, and in 2025 somebody measured it.
What the researchers actually did
The team studied 557 healthy Danish men aged 23 to 92, split into 326 men aged 65 or under and 231 men over 65. They did not ask the men how they felt. They measured the body.
- Appendicular lean mass and skeletal muscle index: the muscle in the arms and legs, scaled to body size.
- Handgrip strength: a squeeze test that predicts more about long term health than it has any right to.
- Leg extension power: force and speed together, not slow grinding strength.
- The 30 second sit to stand test: how many times you can get out of a chair in half a minute.
Then they measured total testosterone, sex hormone binding globulin and albumin, and calculated free testosterone from those three. Every result was adjusted for age, body mass index and body fat percentage. That last adjustment is where the story turns out to be.
Most of your testosterone is not available to you
Testosterone does not float around loose. It travels attached to carrier proteins, mainly sex hormone binding globulin, and while it is held there it does nothing at all. Albumin carries another share, more loosely. Only the small unbound remainder, the free fraction, can enter a cell and act.
Total testosterone counts all of it: tightly bound, loosely bound, free. It is a warehouse inventory. The free fraction is what is on the shelf.
Here is why that matters more as you age. Sex hormone binding globulin tends to rise over the years. More of the hormone gets locked in transit. A man can carry a perfectly respectable total testosterone while the biologically active portion has quietly fallen. The report reassures. The tissue never receives.
What they found
In the men over 65, calculated free testosterone was positively associated with skeletal muscle index, percentage lean mass, grip strength, leg power and sit to stand performance. Total testosterone was not, with the single exception of leg power.
The sharpest detail is what happened once body fat was accounted for. Total testosterone's link to grip strength weakened until it no longer reached statistical significance. Its link to muscle index disappeared altogether. The free testosterone associations held.
When every muscle measure was pooled into one overall picture of muscle status, free testosterone and the free testosterone index were significantly associated with it in the older men. The total was not.
The younger men showed nothing
In the 326 men aged 65 or under, the pooled model found no significant relationship. Either measure, either direction, nothing. That deserves saying out loud, because it is the opposite of a sales pitch. This looks like a problem that arrives with age rather than a universal law about men and hormones. If you are in your thirties and tired, this particular study is not your explanation.
What a study like this cannot do
It cannot show cause. This was a snapshot of men at a single moment, not a group followed through time, so it establishes association and nothing stronger. It does not prove that raising the free fraction builds muscle. Men who carry more muscle may simply run different hormone profiles, and the arrow could point either way.
The limitations are real, so here they are. Blood was drawn without fasting and not exclusively in the morning, which departs from testing guidelines. Testosterone was measured by immunoassay rather than mass spectrometry. Free testosterone was calculated with the Vermeulen equation rather than measured directly by equilibrium dialysis, although the Endocrine Society, the European Society of Endocrinology and the Danish Endocrinology Society all endorse that calculation when direct measurement is impractical, which in ordinary practice it usually is. This is one good study consistent with earlier work. It is not a verdict.
Why it changes what gets measured
Picture the man this study describes. He is in his sixties, weaker than he was, quietly losing ground. His total testosterone comes back inside the range and he is told he is fine. Nobody measured his sex hormone binding globulin, which was the number that would have explained him, and nobody calculated the free fraction, which cannot be worked out without the binding globulin and albumin sitting next to the total.
Nothing exotic is required to avoid that. The three values have to be ordered together and read as one picture, against what the man is actually experiencing. It is a question of how the assessment is built, not of what technology exists. The structure of that assessment is set out in how low testosterone is diagnosed.
This site sells nothing and prescribes nothing. It exists to explain the evidence, including the parts that are inconvenient, and one of those parts is that plenty of men with real symptoms turn out to have a hormone system behaving perfectly well and something else going on entirely. Finding that out properly is worth as much as any treatment.
What is not worth much is a single normal number, read alone, mistaken for an answer.