Total testosterone
is only part of the story.
Most testosterone is bound to SHBG and albumin and unavailable to tissue. This calculates the free and bioavailable fractions from three lab values using the Vermeulen equation, which is what most reference labs report as ‘calculated free T’.
CALCULATED FREE TESTOSTERONE
—How it’s calculated
Vermeulen (1999), binding constants K_SHBG = 1.0 × 10⁹ L/mol, K_alb = 3.6 × 10⁴ L/mol N = 1 + K_alb × [albumin] FT = ( −(N + K_SHBG × ([SHBG] − [T])) + √( (N + K_SHBG × ([SHBG] − [T]))² + 4 × N × K_SHBG × [T] ) ) ÷ (2 × N × K_SHBG) Bioavailable T = FT × N (free + albumin-bound) Unit conversions: total T ng/dL × 0.0347 = nmol/L · albumin g/dL × 10 ÷ 69 000 = mol/L
Assumptions and limits
- Calculated free T agrees well with equilibrium dialysis across the normal range and diverges at extremes of SHBG; it is an estimate, not a measurement.
- Albumin defaults to 4.3 g/dL if you leave it blank, as most labs do.
- Reference ranges differ by lab and assay. A number here is not a diagnosis of deficiency.
References
- A critical evaluation of simple methods for the estimation of free testosterone in serum.J Clin Endocrinol Metab 1999.PubMed 10523012 · doi:10.1210/jcem.84.10.6079
Literature retrieved from PubMed on October 18, 2026, favouring human studies and reviews. Numbered markers in the text show which paper supports which statement; uncited entries are background reading.