Free vs total testosterone - why the gap matters

The three pools of testosterone

When a lab reports "total testosterone," it's measuring every testosterone molecule in the sample - but those molecules are not equally available to your body. Testosterone circulates in three pools:

• SHBG-bound (~40–60%): bound tightly to sex hormone-binding globulin. SHBG grips testosterone with very high affinity, so this fraction is effectively locked away and biologically inactive.
• Albumin-bound (~33–54%): bound loosely to albumin. The bond is weak - four to five orders of magnitude weaker than SHBG - so this testosterone readily dissociates in the capillary bed and is treated as available to tissues.
• Free (~1–4%): completely unbound, diffusing freely into cells. Small in quantity, large in importance.

Two terms fall out of this. Free testosterone is just the unbound slice. Bioavailable testosterone is free plus the albumin-bound portion - everything that isn't trapped on SHBG - which works out to roughly 30–50% of total. Both are attempts to answer the question that total T can't: how much testosterone can your body actually use?

The free hormone hypothesis - why "free" is the number that acts

The governing idea, formalized decades ago and validated experimentally since, is the free hormone hypothesis: biological activity is driven by the unbound hormone concentration, not the total, because only free hormone diffuses across the cell membrane to its receptor. Binding proteins like SHBG act as a reservoir and a gatekeeper. In transgenic mice engineered to over-express human SHBG, total androgen levels rose dramatically - yet free testosterone and actual androgen action on tissues stayed suppressed, a clean confirmation that SHBG-bound testosterone simply can't reach the target.

That's the whole reason a single total-T number can mislead. Two men with an identical total of 500 ng/dL can have very different free testosterone if their SHBG differs - and it's the free value that lines up with how they feel.

SHBG: the dial that sets your free fraction

If free T is what matters, SHBG is what controls it. The more SHBG you have, the more testosterone gets sequestered, and the lower your free fraction - regardless of total. SHBG isn't fixed; a range of conditions push it up or down.

Two patterns are worth internalizing. First, aging raises SHBG, so free and bioavailable testosterone decline faster than total with age - in the Baltimore Longitudinal Study, total T fell ~1.6%/year while free and bioavailable T dropped ~2–3%/year. A 55-year-old with the "same" total T as his 35-year-old self usually has meaningfully less free T.

Second, obesity and insulin resistance lower SHBG (high insulin suppresses the liver's SHBG output). So in heavier men, total T often reads low partly because SHBG is suppressed - while free T can be relatively preserved. That's the mirror image of the high-SHBG problem, and another reason a total-only panel misleads in both directions.

How free testosterone is measured (and the test to avoid)

Not all "free T" results are created equal - the method matters more than people realize.

Equilibrium dialysis - the gold standard

The reference method physically separates free from bound testosterone across a membrane, then measures the free side directly. It's accurate but labor-intensive and not available everywhere, so it's usually reserved for difficult cases.

Calculated free testosterone - the practical standard

The workhorse for everyday use. Plug total T, SHBG, and albumin into the Vermeulen equation and you get a calculated free T that correlates closely with equilibrium dialysis across most conditions. It's fast, cheap, and reliable enough for routine decisions - which is exactly why our free testosterone calculator uses it. The catch: it's only as good as its inputs, so you need SHBG and albumin drawn on the same sample.

Direct analog immunoassays - skip these

The cheap "direct free testosterone" immunoassay is the one to be wary of. The Endocrine Society is blunt about it: clinicians should not use direct analog-based free-T assays because they're inaccurate. In practice they tend to track total T rather than true free T - so they can give you a falsely reassuring (or falsely low) number precisely when SHBG is abnormal and you most need the truth. If your lab report says "free testosterone, direct," treat it with suspicion and ask for a calculated value.

The numbers - what counts as low

Reference ranges are where this gets frustrating, because there is no single harmonized cutoff for free testosterone - it depends on the lab and the assay, so you have to read your result against that lab's own range. For total T, a CDC-standardized study of healthy young men put the middle 95% at roughly 264–916 ng/dL, and the AUA uses <300 ng/dL (on two morning draws, with symptoms) as a working threshold for low T.

For free T, the most-cited anchor comes from EMAS, which paired total T < 11 nmol/L (~317 ng/dL) with free T < 220 pmol/L (~6.4 ng/dL, or ~64 pg/mL) to define symptomatic late-onset hypogonadism. Use that for orientation, not as a hard line - your lab's range and assay are what matter for your result. And remember the core lesson: a normal total with a low calculated free is a real finding, not a lab error.

What to actually do

If you have low-T symptoms, the practical move is to never order total testosterone alone. Draw it fasting, in the morning, alongside SHBG and albumin, then calculate free T. If your total looks normal but you feel off, the calculated free value is often where the answer hides - especially if you're over 45, lean with naturally high SHBG, or carrying extra weight. Confirm any abnormal result on a second morning draw before acting on it, and interpret everything with a clinician. → see the full bloodwork guide for the rest of the panel and lab-day timing.