🧬 Free Androgen Index FAI
The Free Androgen Index estimates bioavailable androgen from total testosterone and SHBG, a practical surrogate for free testosterone in hyperandrogenism workups.
Free Androgen Index FAI
Total testosterone (nmol/L)
SHBG (nmol/L)
When to use
Enter total testosterone and SHBG (both nmol/L); the tool returns FAI. In women, an elevated value supports hyperandrogenism alongside clinical features (hirsutism, acne, oligomenorrhea).
How it works
FAI = total testosterone (nmol/L) × 100 / SHBG (nmol/L). In women FAI is typically < 5 (laboratory-dependent cutoffs vary).
Key points
- FAI is only a surrogate for free testosterone and loses accuracy at very high or very low SHBG; equilibrium dialysis or calculated free testosterone is preferred when precision matters. (original synthesis · not guideline verbatim)
- SHBG falls with obesity, insulin resistance, and hypothyroidism, raising FAI independently of true androgen excess.
- Interpret against the local assay's reference interval and sex; the male range is far higher and the index is not used the same way.
References
- Vermeulen A, et al. A critical evaluation of simple methods for the estimation of free testosterone. J Clin Endocrinol Metab. 1999;84(10):3666-3672.
- Teede HJ, et al. International evidence-based guideline for the assessment and management of PCOS 2023. J Clin Endocrinol Metab. 2023;108(10):2447-2469.
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.