Testosterone lab tests: total vs free vs bioavailable

Three numbers, one hormone — and why clinicians look at all of them.

TL;DR

• Total testosterone counts every molecule in circulation — the headline number.
• Free testosterone (~2% of total) is the part the body can actually use.
• Bioavailable testosterone = free + loosely bound. Often the most useful single number, especially when SHBG is unusual.

What it is

Testosterone in the bloodstream travels three ways. About 60 percent is tightly bound to a carrier protein called SHBG (in plain English: sex hormone binding globulin, a protein the liver makes that grips testosterone like a clenched fist). About 38 percent is loosely bound to albumin. About 2 percent floats free. Think of SHBG-bound testosterone as money in a CD — present but not accessible. Albumin-bound is money in checking. Free testosterone is cash in your wallet.

How it works

Only free and loosely-bound testosterone can leave the bloodstream and act on cells. The total number tells you how much testosterone exists. The free or bioavailable number tells you how much is actually doing work. When SHBG is high (common in older men, those with liver disease, or some thyroid conditions), total testosterone can look normal while free testosterone is functionally low. The opposite is true with low SHBG — total testosterone can look low while free testosterone is fine. The 2018 Endocrine Society guideline recommends measuring SHBG alongside total testosterone when there is clinical suspicion.

Who asks about it

People ask about the different testosterone tests after they get one number from a primary-care lab draw, the result lands in the gray zone, and a clinician suggests follow-up tests. Some ask after a friend mentions “my free T was low even though my total was normal.” Both situations are real and common.

What the research says

Reference ranges for total testosterone in adult men typically span 264 to 916 ng/dL, per the CDC’s harmonized standardization, per Travison et al. 2017. Free testosterone reference ranges vary by lab and assay method — direct immunoassay can give different results than equilibrium dialysis (the gold standard). About 4 in 10 men with total testosterone in the borderline range show a clinically meaningful free testosterone abnormality on follow-up.

What to know before considering it

Testosterone testing should be done early morning (7 to 10 AM) and ideally on two separate days, because levels fluctuate diurnally and day-to-day. A single low result is not a diagnosis. Symptoms must be present alongside two confirmatory low readings before testosterone therapy is considered, per Endocrine Society guidance.

The Halftime POV

We order total, free, and SHBG on the baseline panel for any PRIME-curious reader. One number rarely tells the full story. Two numbers usually do. We discuss the result against the symptoms — not the symptoms against the number — because population reference ranges are not individualized targets.

Related reading:

• Testosterone panels: total, free, and SHBG
• Low testosterone diagnosis: the workup
• Men’s midlife panel 2026

Disclaimer

This article is educational and is not medical advice. Compounded medications are not FDA-approved. Clinical outcomes depend on individual factors and require physician evaluation. Results vary. Halftime Health is launching soon — join the waitlist to get updates.

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Sources

• Travison et al. — Harmonized reference ranges, 2017
• Endocrine Society Clinical Practice Guideline