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Instant Calculation
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Computes free testosterone from total testosterone, SHBG, and albumin using the Vermeulen equation (Vermeulen A, Verdonck L, Kaufman JM. "A critical evaluation of simple methods for the estimation of free testosterone in serum." J Clin Endocrinol Metab. 1999;84(10):3666-72), a widely-used, validated clinical-chemistry method shown in the original paper to correlate strongly (r=0.918) with equilibrium dialysis. It solves the mass-action equilibrium between free, albumin-bound, and SHBG-bound testosterone using standard association constants (Ka=3.6x10^4 L/mol for albumin, Kt=1x10^9 L/mol for SHBG). This is a validated clinical-chemistry lab-value calculation, not a risk-prediction score. For general reference and educational purposes only — not a medical diagnosis. Consult a doctor for actual interpretation of hormone panels.
How it works
Enter total testosterone (nmol/L or ng/dL), SHBG (nmol/L), and albumin (g/dL, default 4.3). The tool solves the Vermeulen quadratic equation for free testosterone.
- Enter total Testosterone.
- Enter total Testosterone unit.
- Enter sHBG (nmol/L).
- Enter albumin (g/dL).
- Click Calculate to see your results.
Examples
Total T 20 nmol/L, SHBG 30 nmol/L, Albumin 4.3 g/dL
Free testosterone ≈ 444 pmol/L (≈128 pg/mL), about 2.2% of total — consistent with the commonly cited ~1-4% free fraction in adult males.
Who should use it
- Reviewing a hormone panel.
- Endocrinology lab-value education.
Industry applications
- Endocrinology reference
- Clinical chemistry education
Advantages
- Validated method with strong correlation to equilibrium dialysis, per Vermeulen et al. (1999).
- Avoids the cost and unavailability of direct equilibrium dialysis testing.
Limitations
- Still an estimate, not a direct physical measurement.
- Association constants are population averages, not individualized.
Common mistakes to avoid
- Entering total testosterone and SHBG in mismatched or incorrect units, which will substantially skew the result.
Best practices
- Use albumin from the same blood draw when available, rather than assuming the default 4.3 g/dL.
Tips
- Free testosterone is typically a small fraction (roughly 1-4%) of total testosterone — most testosterone circulates bound to SHBG and albumin, per standard endocrinology teaching.