Free Androgen Index (FAI) Calculator

Free Androgen Index (FAI) Calculator for Endocrinology. There is no single universal cutoff; each lab must set its own using its testosterone and SHBG assays. In healthy premenopausal women the FAI is typically below about 5 (roughly 5%), and many diagnostic studies place the PCOS/hyperandrogenism threshold in the range of about 4.5 to 7. A higher FAI reflects greater bioavailable androgen and correlates with hirsutism severity, insulin resistance, and adverse metabolic phenotype; in the O'Reilly cohort FAI tracked with total androgen metabolite excretion and dysglycaemia. A single elevated value supports, but does not by itself establish, hyperandrogenism, and a value in the reference range does not exclude it if clinical signs are strong (measure androstenedione or free testosterone).

How this calculator works

The Free Androgen Index estimates the biologically available fraction of circulating testosterone from two routine assays. It is computed as FAI = (total testosterone / SHBG) x 100, with both total testosterone and sex hormone-binding globulin (SHBG) expressed in the same molar units (nmol/L), which makes the index a unitless percentage. Because SHBG binds roughly 60-80% of circulating testosterone with high affinity, dividing by SHBG corrects total testosterone for carrier-protein capacity: a woman with a normal total testosterone but a low SHBG (common with insulin resistance and obesity) can have a distinctly elevated FAI. The index is a surrogate for free/bioavailable testosterone, not a direct measurement.

When to use this calculator

Use the FAI mainly in adult women being worked up for biochemical hyperandrogenism, particularly suspected PCOS, hirsutism, or unexplained oligomenorrhoea, when equilibrium dialysis for true free testosterone is unavailable. It is most useful when total testosterone is borderline but SHBG is suppressed. Do not use it as the primary index when total testosterone is markedly elevated (rule out androgen-secreting tumour or non-classic CAH instead), in pregnancy or with exogenous oestrogen/oral contraceptives (which raise SHBG and distort the ratio), or in men, where free testosterone equations (e.g. Vermeulen) are preferred. It should not be applied to adolescents to make a PCOS diagnosis, per the 2023 international guideline.

Inputs used

  • Total testosterone
  • SHBG

Clinical interpretation

There is no single universal cutoff; each lab must set its own using its testosterone and SHBG assays. In healthy premenopausal women the FAI is typically below about 5 (roughly 5%), and many diagnostic studies place the PCOS/hyperandrogenism threshold in the range of about 4.5 to 7. A higher FAI reflects greater bioavailable androgen and correlates with hirsutism severity, insulin resistance, and adverse metabolic phenotype; in the O'Reilly cohort FAI tracked with total androgen metabolite excretion and dysglycaemia. A single elevated value supports, but does not by itself establish, hyperandrogenism, and a value in the reference range does not exclude it if clinical signs are strong (measure androstenedione or free testosterone).

Worked example

A 28-year-old woman with hirsutism and irregular cycles has total testosterone 2.3 nmol/L (within many lab reference ranges) and SHBG 22 nmol/L. FAI = (2.3 / 22) x 100 = 10.5. Despite a "normal-looking" total testosterone, the low SHBG drives the FAI above a typical upper reference of ~5, supporting biochemical hyperandrogenism and fitting a PCOS picture under Rotterdam criteria when combined with clinical and ultrasound features.

Limitations and safety notes

The FAI is only validated in women; in men and in the low-SHBG range it overestimates free testosterone and correlates poorly with equilibrium-dialysis free testosterone. It is highly sensitive to SHBG, so obesity, insulin resistance, hypo/hyperthyroidism, oestrogens, and androgens shift the index independently of true androgen production. Immunoassay-based total testosterone is unreliable at the low female concentrations, and assay/unit inconsistencies (ng/dL vs nmol/L) are a frequent source of error. Cutoffs are lab- and population-specific and do not transfer between assays or ethnic groups.

Frequently asked questions

What units do I enter, and does it matter?

Enter total testosterone and SHBG both in nmol/L. If testosterone is reported in ng/dL, convert first (ng/dL x 0.0347 = nmol/L). Mixing units is the single most common cause of a wrong FAI.

What is a normal FAI in women?

Most healthy premenopausal women fall below about 5. Diagnostic thresholds for hyperandrogenism cluster around 4.5-7 depending on the assay, so always compare against your own laboratory's reference range rather than a fixed number.

Is FAI better than measuring free testosterone directly?

No. Equilibrium dialysis or a calculated free testosterone (which also uses albumin) is more accurate. FAI is a convenient surrogate that only needs testosterone and SHBG, but it is less reliable at low SHBG.

Can I use FAI in a woman on the oral contraceptive pill?

Interpret with caution. Combined oral contraceptives and other oestrogens raise SHBG substantially, which lowers FAI and can mask true androgen excess. Ideally assess androgens off hormonal contraception for at least three months.

Does an elevated FAI diagnose PCOS?

No. It documents biochemical hyperandrogenism, which is one component of the Rotterdam criteria. PCOS diagnosis requires two of three features (hyperandrogenism, ovulatory dysfunction, polycystic ovarian morphology or elevated AMH) after excluding mimics.

References

  • Teede HJ, Tay CT, Laven JJE, et al. Recommendations from the 2023 international evidence-based guideline for the assessment and management of polycystic ovary syndrome. Eur J Endocrinol. 2023. PMID: 37580861.
  • O'Reilly MW, Taylor AE, Crabtree NJ, et al. Hyperandrogenemia predicts metabolic phenotype in polycystic ovary syndrome: the utility of serum androstenedione. J Clin Endocrinol Metab. 2014;99(3):1027-36. PMID: 24423344.
  • Nadaraja RND, Sthaneshwar P, Razali N. Establishing the cut off values of androgen markers in the assessment of polycystic ovarian syndrome. Malays J Pathol. 2018;40(1):33-39. PMID: 29704382.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: June 2, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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