Total + Free Testosterone (Dialysis)

Most testosterone in blood is bound to carrier proteins. The tightly bound part is not directly available to tissues. The unbound part is small, but it can explain why a standard total result and your symptoms don't always match.

The word dialysis in this panel's name has nothing to do with kidneys. It refers to equilibrium dialysis. This lab technique separates free hormone from protein-bound hormone before measurement. Paired with a direct molecular method for total testosterone, it gives a cleaner answer than antibody-based testing, especially at low levels.

What This Panel Reveals

The total number tells you how much testosterone is in circulation, bound and unbound together. It is the right starting point, and it is the number most published reference ranges were built on. What it can't tell you is how much of that supply is freely available.

That depends on a carrier protein your liver makes. Labs call this protein sex hormone binding globulin. The abbreviation is SHBG. Albumin is the main protein in blood. It holds testosterone more loosely. On average, about two percent of testosterone circulates unbound.

SHBG is not fixed. Aging and liver disease raise it. Excess weight and insulin resistance lower it. Thyroid disease and certain medications move it as well, which is why guidelines single it out as a reason to measure or calculate free testosterone instead of relying on total testosterone alone.

The cost of ignoring this is measurable. In the European Male Ageing Study, using total testosterone alone instead of total plus calculated free testosterone produced false-positive findings in 23.1% of men with low total testosterone and false-negative findings in 9.5% of men with normal total testosterone. That study used calculated free testosterone, not equilibrium dialysis, so it supports the total-plus-free logic rather than proving this exact method. Still, the direction matters: men with normal total and low free testosterone had more signs of deficiency, while men with low total and normal free testosterone largely did not.

Why the Method Matters

Most testosterone testing is done with antibody-based methods. These get shakier as concentrations fall. In young men with obesity, an antibody-based test classified 53.7% as testosterone deficient. A direct molecular method on the same population put it at 26.3%. That is roughly twice as many men told they have a problem.

Free testosterone has the same problem, worse. The Endocrine Society says not to use direct analog free-testosterone tests because they are inaccurate. The usual workaround is to calculate free testosterone from total testosterone, SHBG, and albumin. That is useful, but it is still an estimate. In one comparison, the common Vermeulen formula ran about a fifth higher than equilibrium dialysis. This panel skips that estimate.

European lab practice still varies. In a 2025 survey, only one in four centers used a direct molecular method for total testosterone, and free testosterone was usually reported as a calculation. Reference ranges also varied across labs. So the method is not a footnote. It changes the answer.

How to Read Your Results Together

When the two numbers disagree, the free value is the clue to follow, then check SHBG to learn why. Total is a supply number. Free is closer to the available-to-tissue number.

What to Do with Your Results

A single low result is weaker evidence than it looks. Testosterone moves day to day, and guidelines call for confirming a low value on a second fasting morning sample before calling it deficiency. Do that before drawing conclusions or starting anything.

If the repeat confirms it, the next question is where the problem starts. Luteinizing hormone and follicle-stimulating hormone are the two signals the pituitary sends to the testicles. The pituitary is the small hormone-control gland at the base of the brain. High signal levels alongside low testosterone point toward the testicles. Low or normal signal levels alongside low testosterone point upstream. Check prolactin, and get medical evaluation if prolactin is high, the signal hormones are very low, or symptoms suggest a pituitary problem. Adding SHBG is useful too, since it explains the gap between your two testosterone numbers.

If you are monitoring testosterone therapy, guidelines use testosterone and hematocrit checks at 3 to 6 months, at 12 months, and then yearly. If you are tracking changes in weight, training, sleep, alcohol, or a medication known to suppress testosterone, retest after the change has had time to settle. Use the same morning window and the same lab, because free testosterone ranges are not standardized across labs the way total ranges are.

When Results Can Be Misleading

Time of day moves both numbers at once. In men in their thirties, a 4pm draw runs 20% to 25% below an 8am draw, and by age 70 the gap narrows to about 10%. In one study, half the men who fell below the deficiency cutoff on an afternoon sample were normal at every morning visit. Draw in the morning, fasting.

Acute illness, recovery from illness, and short courses of drugs that suppress testosterone, opioids especially, all pull results down. Guidelines say not to test during any of them. Wait until you are back at baseline.

If you have kidney disease, the accuracy argument gets stronger rather than weaker. In people on kidney dialysis, automated antibody-based tests overestimated testosterone by as much as 92% compared with a direct molecular method. Unusual blood chemistry is exactly what can throw antibody-based testing off.