How to estimate free testosterone from your labs
Most testosterone in blood is bound to SHBG and albumin; only a small fraction is free. The Vermeulen equation estimates that free fraction from three lab values you already have on a report.
- Enter total testosterone in ng/dL or nmol/L from your lab report.
- Enter SHBG in nmol/L (sex hormone–binding globulin).
- Enter albumin in g/dL — or use the shown default if it is not on your panel.
- Read free and bioavailable T plus the free percentage, then discuss the numbers with your provider.
Worked example: total T 600 ng/dL, SHBG 30 nmol/L
A total testosterone of 600 ng/dL with SHBG 30 nmol/L and albumin 4.3 g/dL, via Vermeulen:
- Free testosterone — ≈ 12 ng/dL (about 2% of total).
- Bioavailable testosterone — ≈ 280 ng/dL (free plus the albumin-bound fraction).
- Why it matters — the same total T gives a lower free fraction when SHBG is high, and a higher one when SHBG is low.
These figures depend on your exact inputs and the assumed binding constants (shown in the tool). Reference ranges vary by lab — interpret any result with your provider.
Frequently asked questions
What is free testosterone?
Most testosterone circulates bound to SHBG (tightly) and albumin (loosely); the small unbound portion is free testosterone. Together, free plus albumin-bound is called bioavailable testosterone. The Vermeulen equation estimates these from total T, SHBG, and albumin.
What is the Vermeulen equation?
It is a widely used formula that models how testosterone binds to SHBG and albumin to estimate the free fraction, using published association constants. It is an estimate — an equilibrium-dialysis lab measurement is the reference method. The tool shows the constants it uses.
What units should I enter?
Total testosterone in ng/dL or nmol/L, SHBG in nmol/L, and albumin in g/dL. If albumin is not on your panel, a normal default is commonly used. Use the exact values from your own lab report.
Is a given free-testosterone number normal?
Reference ranges differ between labs and change with age, so this tool does not label a value normal or abnormal. It is an educational estimate, not a diagnosis — review your results with a licensed provider.
Is this free testosterone calculator medical advice?
No. It estimates from the labs you enter and shows its work; it never diagnoses or recommends treatment. Always interpret lab results with your licensed provider.
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