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Free Testosterone: The Number That Actually Matters

Total testosterone is the headline number. Free testosterone is the one that does the work. If you've only ever seen total T on a result, you're missing the read that explains why men with 'normal' totals can still feel awful.

What does 'free' testosterone actually mean?

About 98% of circulating testosterone is bound — mostly to SHBG, partly to albumin. The remaining 1-2% is unbound and immediately bioavailable. That's free testosterone.

Loosely-bound albumin testosterone is also active in practice, so 'bioavailable testosterone' (free + albumin-bound) is sometimes used as the working number.

Why do direct free-testosterone assays mislead?

Most labs offer 'direct free testosterone' on the order menu. The Endocrine Society advises against it — the assay is unreliable at the low end where it matters most.

The gold standards are equilibrium dialysis (research-grade, expensive) and calculated free testosterone (Vermeulen formula: total T + SHBG + albumin). Calculated free T is what FORM uses by default — accurate enough, available anywhere.

What free-testosterone ranges actually matter?

Calculated free testosterone reference ranges vary, but a working frame: above 250 pmol/L is generally fine; 180-250 pmol/L is borderline; below 180 pmol/L is functionally low regardless of total T.

Symptoms (libido, morning erections, energy, recovery, mood) are more useful than any single number. The Endocrine Society guidance is to treat low T when you have both low numbers AND consistent symptoms.

FAQs

Is free testosterone or total testosterone more important?
Free, in most cases. Total tells you how much you have; free tells you how much your body can use. Men with high SHBG can have normal total and functionally low free T.
Why do labs offer direct free T if it's unreliable?
Because it's cheap and commercially available. Endocrine Society guidance is to use calculated free T or equilibrium dialysis instead.

References

  1. [1]Testosterone Therapy in Men with Hypogonadism: Endocrine Society Clinical Practice GuidelineEndocrine Society / JCEM (2018)
  2. [2]Diagnosis and Management of Male Late-Onset HypogonadismPubMed / J Clin Endocrinol Metab

Get a panel that calculates free T properly — not the unreliable direct assay.

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