Free Testosterone
Equilibrium Dialysis (Gold Standard) vs Calculated (Vermeulen) vs the Discredited Direct Analog Immunoassay
Evidence grade 4 of 5, Strong. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- A laboratory measurement. Equilibrium dialysis is the reference method; the calculated (Vermeulen) estimate is the practical standard; the direct analog immunoassay is discredited.
Reference only. Not medical advice.
What can go wrong
A direct-analog free-T result can both falsely reassure and falsely alarm, and because it drifts with SHBG it is least trustworthy in exactly the high- and low-SHBG patients for whom free T was ordered in the first place. Even the calculated value can mislead if the underlying total T came from a non-standardized immunoassay or a mistimed draw - errors in the inputs propagate into the estimate.
This is something to take to a clinician. The Atlas can tell you what is known about free testosterone in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
Free testosterone is the 1-3% of circulating testosterone not bound to any protein. It matters most when SHBG is abnormal, because total T can then mislead. There are three credible approaches and one to avoid: equilibrium dialysis (the gold-standard direct measurement), calculation from total T, SHBG, and albumin using the Vermeulen equation (the practical standard, validated against dialysis at r about 0.99 in Vermeulen 1999), and the direct analog/tracer immunoassay - which multiple expert bodies consider inaccurate and unreliable and recommend against. Many cheap panels still run the discredited direct assay.
The fraction that actually reaches tissue - measured well three ways and badly a fourth.
What people take it for
What it is taken for. Approval, where it exists, covers only some of these.
How it works
Only unbound testosterone diffuses freely into cells, so free T is the most direct index of biologically available hormone. Equilibrium dialysis physically separates free hormone across a membrane and quantifies it, ideally by LC-MS/MS. The Vermeulen calculation applies the law of mass action with published binding constants for SHBG and albumin to estimate free T from three measured inputs. The direct analog immunoassay uses a labeled testosterone analog and assumes it partitions like native hormone - an assumption that fails, producing values that track SHBG artifactually rather than true free T.
The catch: 'free T' means nothing without the method
The phrase 'free testosterone' on a lab report can denote three completely different measurements of differing quality. A calculated free T is only as good as its SHBG and albumin inputs, and different calculators use slightly different binding constants, so values are not perfectly interchangeable across labs. A direct-analog free T is cheap, common, and considered unreliable by the Endocrine Society and clinical chemistry bodies. Consumers almost never see which one they bought.
What people get wrong
Often saidFree T is a more advanced, more accurate number than total T
ActuallyOnly if measured or calculated properly. The commonly bundled direct analog immunoassay is discredited, and a calculated value inherits every error in its total-T and SHBG inputs.
Often saidA free-T number can be compared straight across labs
ActuallyCalculated values depend on which equation and binding constants a lab uses, and dialysis vs calculation vs analog assay are not interchangeable.
Who profits, and who pushes back
William Rosner, MD
The direct analog free-testosterone assay is not an acceptable method; it does not measure what it claims to measure.
What we do not know
- How many consumer 'free T' panels still run the direct analog immunoassay?
- Do calculated free-T values from different DTC labs agree closely enough to compare over time?
At a glance
- Free fraction
- ~1-3% of total testosterone
- Gold standard
- Equilibrium dialysis (or ultrafiltration)
- Practical standard
- Calculated (Vermeulen 1999 equation)
- Inputs for calculation
- Total T, SHBG, albumin
- Method to avoid
- Direct analog / tracer immunoassay
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about Free Testosterone. A short entry is not a safe entry.
- Timeline. We have not built the dated record of research and regulatory events.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
- Where we land. We have not taken a position on this yet.
Sources
The studies below are named and described, but not linked. You cannot open them from here and check our reading against them, which means this page is our summary of the literature rather than a document you can verify. Weigh it accordingly until the links exist.
The studies that matter
Vermeulen, Verdonck, Kaufman, JCEM1999
Derived and validated the calculated free-testosterone equation; it correlated with equilibrium dialysis at about r=0.987, far better than the direct analog immunoassay
Rosner et al., position statement on testosterone assays, JCEM2007
Concluded direct analog free-testosterone immunoassays are inaccurate and should not be used; endorsed equilibrium dialysis or validated calculation
Bhasin et al., Endocrine Society guideline2018
Recommends free testosterone by equilibrium dialysis or accurate calculation when total T is near the threshold or SHBG is abnormal