Free androgen index optimal range and reference range

Compiled by Alina Radu, founder of Sera, from the published sources below · Last reviewed November 9, 2026

Sera's bands for Free androgen index, which differ for men and women:

  • Female: Optimal 1 – 4; sufficient 0.3 – 5.6; out of range below 0.3 or above 5.6.

Evidence grade: Published reference interval. A band that primarily reflects a published adult laboratory or population reference interval. It is not necessarily applicable to every assay or laboratory.

All units

UnitApplies toOptimalSufficientOut of range
ratioFemale1 – 40.3 – 5.6below 0.3 or above 5.6

What the evidence says

The free androgen index is total testosterone divided by SHBG, times 100, and is used in women as a rough estimate of free testosterone when looking for androgen excess, as in PMOS (formerly polycystic ovary syndrome). The women's acceptable band of 0.3 to 5.6 is one NHS laboratory's reference interval for ages 20 to 49; other laboratories print upper limits of 7 to 11, because the result depends on the testosterone and SHBG assays used, and guidelines give no single cut-off. The narrower optimal band of 1 to 4 is a functional target with no published threshold behind it. Sera does not score the index in men, where laboratories do not report it: use total testosterone, with calculated free testosterone where SHBG is abnormal.

Sources

  • Free Androgen Index — York and Scarborough Teaching Hospitals NHS Foundation Trust, 2026 · Laboratory / reference-interval sourceWomen aged 20 to 49: 0.3 to 5.6%; over 50: 0.2 to 3.6%. The index is reported for women only. Matches Sera's female sufficient band exactly. Page undated; current test directory.
  • Free Androgen Index (FAI) — Gloucestershire Hospitals NHS Foundation Trust, 2025 · Laboratory / reference-interval sourceWomen aged 20 to 50: 0.18 to 7.07%; over 50: 0.10 to 4.95%. Valid only for the local Roche assays and a normal albumin. No male range. Shows the upper limit is assay-specific.
  • Free androgen index — North Bristol NHS Trust (Severn Pathology), 2026 · Laboratory / reference-interval sourceWomen aged 18 to 46: 0.7 to 10.9%; 47 and over: 0.2 to 6.8%. Calculated for women only. A third laboratory with a different upper limit. Page undated; current test directory.
  • Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome — Teede HJ, Tay CT, Laven JJE, et al. — The Journal of Clinical Endocrinology & Metabolism, 2023 · Clinical guidelineThe index may be used to estimate free testosterone when assessing biochemical hyperandrogenism in women. No numeric cut-off is given, and reference ranges are said to vary widely between methods and laboratories.

What FAI (Free Androgen Index) measures

An estimate of how much active testosterone is available to the body, worked out from total testosterone and SHBG. Because most testosterone is bound to SHBG and inactive, FAI gives a clearer picture of usable androgen than total testosterone on its own. The name stands for Free Androgen Index.

Calculated: (Total Testosterone / SHBG) × 100 (both in nmol/L)

Related hormone health markers

See Free androgen index in the full list of biomarker ranges

About the author

Alina Radu is the founder of Sera and a software engineer. The idea started with her own Hashimoto's tests, scattered across years without the big picture. She is not a clinician: these ranges are compiled from the published sources linked on each page, with every source named. Alina Radu on LinkedIn

This page is a reference for practitioners, not medical advice. Sera's bands are practitioner targets for structured review, not diagnostic thresholds, and Sera does not diagnose or recommend treatment. Read a result alongside the interval printed by the issuing laboratory and the client's history and symptoms. Range set: Sera Production, version 1.0, last reviewed November 9, 2026.