Calculate free androgen index from total testosterone and SHBG, or reverse-calculate testosterone or SHBG for a selected FAI value.

Use total testosterone and SHBG from the same sample. FAI is a screening ratio, not measured free testosterone or a diagnosis.

Your laboratory FAI interval
Optional; enter both limits exactly as reported.

Free Androgen Index Formula

FAI = 100 × Total Testosterone (nmol / L) / SHBG (nmol / L)
  • FAI = a scaled dimensionless index, often displayed with a % sign; not the percentage of testosterone unbound
  • Total Testosterone = serum total testosterone in nmol/L
  • SHBG = sex hormone-binding globulin in nmol/L

Both values must come from the same blood draw. If your testosterone is reported in ng/dL, divide by 28.84 to get nmol/L. If it is in ng/mL, multiply by 3.467. FAI is a screening ratio used mainly in assessing androgen excess in women. It is less accurate when SHBG is below 30 nmol/L and has limited clinical value in men. It is not a measured free-testosterone concentration or a diagnosis.

Rearranged forms used by the reverse mode:

Total Testosterone (nmol / L) = FAI × SHBG / 100; SHBG (nmol / L) = 100 × Total Testosterone (nmol / L) / FAI

Reference Interval and Interpretation Limits

Cutoffs vary by lab and assay. Always compare your result to the reference interval printed on your own report.

Group FAI interpretation Notes
Adult female Use the laboratory interval Intervals depend on assay, age and menopausal status
Adult male No universal cutoff applied FAI has limited clinical value in men; seek laboratory guidance
PCOS suspicion No diagnostic threshold here PCOS assessment requires clinical criteria and exclusion of other causes

Testosterone Unit Conversions

From To Multiply by
ng/dL nmol/L 0.0347
ng/mL nmol/L 3.467
nmol/L ng/dL 28.84

Worked Example

Total testosterone is 2.1 nmol/L and SHBG is 45 nmol/L.

FAI = 100 × 2.1 / 45 = 4.67%

Compare 4.67 with the reporting laboratory's applicable reference interval. This number alone does not establish or exclude androgen excess or PCOS.

FAQ

Why is testosterone converted to nmol/L? SHBG is reported in nmol/L. Both values must use the same units for the ratio to be dimensionless and the percentage to be meaningful.

Is FAI the same as free testosterone? No. FAI is a calculated ratio, not a measurement. Free testosterone may be assessed by equilibrium dialysis or a validated calculation using total testosterone, SHBG and albumin. These methods have their own analytical and clinical limitations.

When is FAI unreliable? FAI can overestimate free androgens when SHBG is below 30 nmol/L and has limited clinical value in men. Pregnancy, oral estrogens and other SHBG-altering conditions require clinical interpretation. Discuss an appropriate free-testosterone assessment with the laboratory or clinician.

What if my lab gives a different reference range? Use your lab's range. Assay methods differ enough that a single universal cutoff does not exist.