🧬 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.
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)
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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.
Worked calculation
The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.
| Total testosterone | 2.0 nmol/L |
|---|---|
| SHBG | 40 nmol/L |
→FAI5
- Formula：Total testosterone × 100 / SHBG
Frequently asked questions
- What is 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.
- How is Free Androgen Index FAI calculated? What is the core formula?
- FAI = total testosterone (nmol/L) × 100 / SHBG (nmol/L). In women FAI is typically < 5 (laboratory-dependent cutoffs vary).
- When is Free Androgen Index FAI used?
- 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).
- What are the key clinical points for Free Androgen Index FAI?
- 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.
- What are the limits and cautions when using Free Androgen Index FAI?
- For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
- How is Free Androgen Index FAI calculated in practice? Can you show a worked example?
- Inputs: Total testosterone 2.0 nmol/L, SHBG 40 nmol/L → Result: FAI 5（Formula: Total testosterone × 100 / SHBG）