Free testosterone by the Vermeulen equation
Calculates free testosterone from total testosterone, SHBG and the assumed albumin.
When to use
Suspected altered free fraction when total testosterone and SHBG do not explain the picture and a direct assay is unavailable.
Pearls and pitfalls
- The equation assumes albumin at 43 g/L, so a markedly abnormal albumin limits the result.
- SHBG enters as the binding capacity measured by immunoassay, not as a value back-calculated from another method.
- The constants are Kt of 1e9 L/mol for SHBG and Ka of 3.6e4 L/mol for albumin, which gives N equal to 22.
- The result is a calculated estimate and does not replace spectrometry or dialysis when the value is decisive.
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Formula
Free testosterone = (−b + square root of b² + 4 × 22 × T) divided by 44, with b = SHBG − T + 22 and concentrations in nmol/L.
Evidence
Binding equation derived from the SHBG and albumin association constants, compared with dialysis and ammonium sulfate methods.
References
- 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):3666-3672. doi:10.1210/jcem.84.10.6079
Frequently asked questions
What is Free testosterone by the Vermeulen equation?
Calculates free testosterone from total testosterone, SHBG and the assumed albumin. Suspected altered free fraction when total testosterone and SHBG do not explain the picture and a direct assay is unavailable.
How is the result calculated?
Free testosterone = (−b + square root of b² + 4 × 22 × T) divided by 44, with b = SHBG − T + 22 and concentrations in nmol/L.
How should the result be interpreted?
Binding equation derived from the SHBG and albumin association constants, compared with dialysis and ammonium sulfate methods.
What are the sources?
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):3666-3672.