Clinical tools · endocrine
Free & bioavailable testosterone calculator
Estimates free and bioavailable testosterone from total testosterone, SHBG and albumin using the Vermeulen mass-action binding equation.
Measured values
Enter results from the same specimen.
4.3 g/dL is the conventional default where albumin was not measured. Enter the measured value where available — it matters in nephrotic syndrome, cirrhosis and pregnancy.
Both are in published use. The choice shifts the result; state which was used when reporting a calculated value.
Calculated
FREE TESTOSTERONE
BIOAVAILABLE TESTOSTERONE
SHBG-BOUND
The equation, and what it assumes
Testosterone circulates in three states: bound tightly to SHBG, bound weakly to albumin, and unbound. The SHBG-bound fraction is not available to tissue. Free plus albumin-bound is the bioavailable fraction.
Vermeulen derived the distribution from the law of mass action using the measured association constants, and it agrees closely with equilibrium dialysis when total testosterone, SHBG and albumin are each measured on reliable assays.
What it assumes. Each SHBG dimer binds two testosterone molecules with equal affinity and no allostery between sites. That assumption has been challenged — work using isothermal titration calorimetry describes complex allostery between the binding sites, and calculated values can diverge from equilibrium dialysis. The equation remains the approach the Endocrine Society's expert panel identified as the best available estimate from reliably measured inputs.
What it does not do. It does not correct for an unreliable total testosterone. A calculated free testosterone inherits every error in its inputs, which is the argument for measuring total testosterone and SHBG by methods accurate at the concentrations that matter.
Measured inputs
A calculated value inherits every error in its inputs. These are the assays the result rests on.