Free & Bioavailable Testosterone Calculator

Free & Bioavailable Testosterone Calculator for Endocrinology. Free and albumin-bound testosterone are distinct from the SHBG-bound fraction. This implementation identifies its albumin molecular-mass convention as 69000 g/mol; another convention can change the estimate slightly. No empirical shortcut, assumed default albumin or automatic diagnostic band is used.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

The independently written binding calculation follows the documented Ghent/ISSAM mass-action convention. In molar units, N = 1 + 36000 x albumin(g/L)/69000 and total testosterone = N x free testosterone + 1e9 x SHBG x free testosterone / (1 + 1e9 x free testosterone). The positive quadratic solution gives free testosterone; N times free gives bioavailable testosterone.

When to use this calculator

Use for an explicit calculated-binding estimate when total testosterone, SHBG and measured albumin are available and the model assumptions are appropriate. Compare with method-specific laboratory interpretation. A calculated fraction is neither a direct measurement nor sufficient to diagnose androgen deficiency or excess.

Inputs used

  • Total testosterone in nmol/L or ng/dL
  • SHBG in nmol/L
  • Measured albumin in g/L or g/dL

Clinical interpretation

Free and albumin-bound testosterone are distinct from the SHBG-bound fraction. This implementation identifies its albumin molecular-mass convention as 69000 g/mol; another convention can change the estimate slightly. No empirical shortcut, assumed default albumin or automatic diagnostic band is used.

Worked example

Total testosterone 10 nmol/L, SHBG 40 nmol/L and albumin 43 g/L give free testosterone approximately 0.173884 nmol/L, or 5.011 ng/dL using 0.0347. Bioavailable testosterone is approximately 4.0749 nmol/L, or 117.433 ng/dL. The source's rounded explanation reports approximately 5.02 and 118 ng/dL.

Limitations and safety notes

Binding constants are model assumptions; assay error and competing SHBG-bound steroids can materially affect results. The source warns about contexts such as pregnancy or competing steroid exposure. Extreme inputs are not clinically validated simply because the arithmetic remains finite. Clinical diagnosis requires appropriate symptoms, sampling and confirmation.

Frequently asked questions

Is calculated free testosterone the same as measured free testosterone?

No. It is a model-derived estimate whose accuracy depends on measurements and binding assumptions.

Why report the albumin mass convention?

It determines the molar albumin concentration and therefore the estimate. Stating it makes comparison with another calculator reproducible.

References

  • Ghent/ISSAM primary calculation explanation: KA 36000 L/mol, albumin molecular mass 69000 g/mol, SHBG association constant 1e9 L/mol and worked equilibrium example. https://www.issam.ch/freetesuit.htm
  • Primary calculator attribution and applicability warning for competing SHBG-bound steroids; calculated fractions are estimates, not measured free testosterone. https://www.issam.ch/freetesto.htm

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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