How this calculator works
This page solves the mass-action model described by Vermeulen and colleagues using the constants in the Ghent group's explanation: albumin binding constant 3.6 × 10⁴ L/mol, SHBG binding constant 10⁹ L/mol and albumin molecular mass 69,000 g/mol. It uses the published equivalence 288.4 ng/dL = 10 nmol/L. Bioavailable testosterone is the free plus albumin-bound portion within this model. It is a calculated estimate, not an equilibrium-dialysis measurement or a diagnosis. The authors caution against reliance on the model during pregnancy or substantial interference from other SHBG-binding steroids. No reference interval or treatment threshold is inferred. The result is an estimate for information only and is not medical advice.
Formula
Let A = 1 + 36,000 × albumin(g/L)/69,000. With testosterone and SHBG in nmol/L, A F² + (A + SHBG - T)F - T = 0. Use the positive root for free testosterone F. Bioavailable T = A F. In reverse, T = A F + SHBG F/(1 + F).
Worked example
For albumin 43 g/L, SHBG 40 nmol/L and total testosterone 10 nmol/L, the model gives about 0.174 nmol/L free and 4.08 nmol/L bioavailable testosterone. With albumin 46 g/L, no SHBG and total 25 nmol/L, A = 25, so free testosterone is 1 nmol/L and the bioavailable total is 25 nmol/L.
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Frequently asked questions
Why specify albumin molecular mass?
Different implementations use different albumin constants. This page explicitly uses the 69,000 g/mol value in the cited Ghent explanation so the convention is reviewable.
Is bioavailable testosterone the same as free?
No. In this model it includes free testosterone plus the fraction weakly bound to albumin.
Can a calculated result choose a treatment?
No. Assay quality, clinical findings and interference matter. Discuss interpretation with a qualified clinician.