Endocrinology
Free & Bioavailable Testosterone Estimator
This calculator uses a mass action equilibrium model (Vermeulen approximation) to derive free and albumin-bound testosterone from total testosterone, sex hormone-binding globulin (SHBG), and albumin.
Used for interpretation thresholds.
Free testosterone
0 pg/mL
0.02 ng/dL · 0.00 nmol/L
Free % of total: 0.00%
Bioavailable testosterone
549.3 ng/dL
19.06 nmol/L (free + albumin-bound)
Free testosterone below typical range. Correlate with symptoms and repeat testing or endocrine evaluation.
How to Use This Calculator
Obtain lab measurements
Use morning total testosterone (preferably LC-MS), SHBG, and serum albumin to ensure accuracy.
Enter values and review results
The tool estimates free and bioavailable testosterone, helping interpret borderline total testosterone results.
Correlate with clinical findings
Always interpret lab values with patient symptoms, comorbidities, and guideline recommendations.
Formula
Free T = Total T ÷ (1 + Ka·[Albumin] + Kshbg·[SHBG])
Albumin-bound T = Ka·[Albumin]·Free T
Bioavailable T = Free T + Albumin-bound T
Constants: Ka (albumin) = 3.6×10⁴ L/mol, Kshbg = 1×10⁹ L/mol. Albumin converted to mol/L (g/dL × 10 ÷ 66.5).
Full Description
Free testosterone is the biologically active fraction not bound to SHBG or albumin. Bioavailable testosterone includes free and albumin-bound portions. This calculator applies a simplified equilibrium model (Vermeulen) widely used in clinical practice when direct equilibrium dialysis is unavailable.
Factors such as obesity, thyroid status, and medications alter SHBG, affecting free testosterone despite normal total levels. Consider repeating testing and using consistent methods for longitudinal monitoring.
Frequently Asked Questions
How accurate is this versus equilibrium dialysis?
Calculated values correlate well but may differ in extreme SHBG states. Use gold-standard assays if results conflict with clinical presentation.
Do I need to adjust albumin in hypoalbuminemia?
Yes. Enter the patient’s actual serum albumin. Hypoalbuminemia increases free testosterone at a given total level.
Can women use this calculator?
Yes. Reference ranges are lower; ensure measurements use a sensitive assay (LC-MS/MS) for accuracy in females and children.
How should results guide therapy?
Follow endocrine society guidelines; treat only when biochemical results align with symptoms and underlying etiology is addressed.