Free Androgen Index (FAI)
A calculated estimate of how much testosterone is free to act, found by dividing total testosterone by SHBG.
AI-drafted. Awaiting verificationAlso called FAI, cFAILOINC 24125-7
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The usual range for FAI depends on sex. Choose female or male above to see where you stand.
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What it is
The free androgen index (FAI) is a calculation, not a direct measurement. Most testosterone in blood is held by a protein called sex hormone-binding globulin (SHBG), and only the rest is free to act. FAI divides total testosterone by SHBG, with both in nmol/L, and multiplies by 100. A lab or app works it out from two results from the same blood draw.
Why it matters
FAI is used mostly in women, to help look into signs of extra androgens such as acne, extra facial or body hair, or irregular periods. The 2023 international PCOS guideline says free testosterone can be estimated by the FAI. In one study, FAI and calculated free testosterone were among the best predictors of polycystic ovary syndrome (PCOS). FAI is less reliable in men. It can overestimate free testosterone when SHBG is low, and other calculations that also use albumin are often preferred.
If it’s low
A low FAI means testosterone is low compared with SHBG. In women this is common and often has a simple explanation. High SHBG, from estrogen in birth control or an overactive thyroid, can lower it. In men, a low FAI can reflect low testosterone or high SHBG, but FAI is a rough guide, and a measured or calculated free testosterone is more reliable.
Common causes
- High SHBG, for example from estrogen in birth control pills or pregnancy
- An overactive thyroid, which raises SHBG
- Lower testosterone production by the ovaries, adrenal glands or testes
- Older age, especially in men, when SHBG tends to rise
What can help
- Ask your clinician whether birth control, hormone therapy, a medicine or your thyroid could explain it
- Ask whether a calculated or measured free testosterone would give a clearer picture
- Tell whoever reads your results about any hormones or supplements you use
If it’s high
A high FAI means testosterone is high compared with SHBG. It is commonly seen in PCOS. It can also happen because SHBG is low, which is common with extra body weight or insulin resistance, so a high FAI does not always mean testosterone itself is high. Your clinician can look at total testosterone and SHBG separately and put the number in context.
Common causes
- Polycystic ovary syndrome (PCOS)
- Low SHBG, for example with extra body weight or insulin resistance
- Testosterone or anabolic steroid use
- Conditions of the adrenal glands that raise androgens
- Rarely, a tumor of the ovary or adrenal gland that makes androgens
What can help
- Tell your clinician about any testosterone, steroids, supplements or hormone medicines you use
- Ask whether total testosterone, SHBG and other androgens should be reviewed alongside the FAI
- Ask whether your result was based on a mass spectrometry testosterone test
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on sex. Choose women or men to see the one that applies.
- Women, 18+, premenopausal0.6–4.4 index (no unit)
Source: Clinica Chimica Acta
What can affect your result
- Total testosterone, which depends on sex, age, time of day and the test method
- SHBG, which rises with estrogen, thyroid hormone and age and falls with extra body weight, insulin resistance and androgens
- Testosterone medicines, anabolic steroids and hormone treatments
- Units: the calculation needs nmol/L for both values
- Whether the lab multiplies by 100, since some reports leave it out
- Anything that changes total testosterone or SHBG changes the FAI. Estrogen, including birth control pills, and an overactive thyroid raise SHBG. Extra body weight, insulin resistance, testosterone and other androgens lower it.
- Direct immunoassays for testosterone are less accurate at the low levels seen in women. The 2023 PCOS guideline recommends mass spectrometry (LC-MS/MS) for total and free testosterone.
- High-dose biotin supplements can interfere with some immunoassays, including some testosterone and SHBG tests. Ask your lab or clinician whether to pause biotin before the draw.
- Both values must be in nmol/L for the calculation. Mixing in ng/dL gives a wrong answer, and some reports omit the multiplication by 100.
FAI is worked out from a total testosterone and an SHBG result from the same blood draw. Testosterone guidance asks for a fasting sample taken in the early morning (about 7 to 10 a.m.), and the same applies here. SHBG shifts gradually, so changes in weight, thyroid status or hormone medicines show up in the FAI over weeks rather than days.
When to talk to a clinician
Talk with a clinician if you notice fast-growing facial or body hair, scalp hair loss, a deepening voice, or periods that have become very irregular or stopped. Men can talk with a clinician about low sex drive, erection changes or ongoing fatigue.
Questions you could ask
- Was my testosterone measured by mass spectrometry or an immunoassay?
- Should I use the same lab for repeat tests?
- Is my FAI high because testosterone is high, or because SHBG is low?
- Which other tests would help put this in context?
- Could birth control, thyroid levels or a medicine explain a low result?
- Would a free testosterone result be more useful than the FAI?
Questions people ask
How is FAI calculated?
Divide total testosterone by SHBG, with both in nmol/L, then multiply by 100. If your testosterone is in ng/dL, divide it by 28.84 first to get nmol/L.
Is FAI the same as free testosterone?
No. FAI is a rough stand-in for free testosterone. Free testosterone can be measured directly or calculated with a formula that also uses albumin, and those are usually more accurate, especially in men or when SHBG is low.
Why does my lab's FAI range differ from what I read online?
FAI depends on how the lab measures testosterone and SHBG, and labs set their own ranges. In one research study of healthy women, the interval was 0.6-4.4 with an immunoassay and 0.4-2.9 with mass spectrometry. Use the range printed on your report.
Show the science and sources
For clinicians and researchers
FAI = total testosterone (nmol/L) / SHBG (nmol/L) x 100. It is a surrogate for free testosterone that assumes all SHBG binds testosterone in the same way. Free testosterone can also be calculated from total testosterone, SHBG and albumin, which is generally preferred over FAI in men: in 126 healthy men aged 20-45, FAI correlated poorly with calculated free testosterone (r squared 0.21 to 0.46) and overestimated it at low SHBG (Ho 2006). In 25 healthy ovulatory women (Bui 2015), the reference interval for FAI was 0.4-2.9 with LC-MS/MS testosterone and 0.6-4.4 with a second-generation immunoassay, and FAI and calculated free testosterone were the strongest predictors of PCOS in 44 patients. The 2023 international PCOS guideline recommends total and free testosterone for biochemical hyperandrogenism, notes that free testosterone can be estimated by the calculated FAI, and recommends LC-MS/MS over direct immunoassays for testosterone. SHBG is lower in obesity, partly through inflammatory signals that reduce its production in the liver (Hammond 2016), so FAI tends to rise with weight independently of testosterone. Some published reports calculate the ratio without the factor of 100, which gives values 100 times smaller, so check which convention a lab uses. Ranges are lab- and method-specific.
LOINC 24125-7 (primary). Specimen: calculated.
Sources
- Testosterone, free testosterone, and free androgen index in women: Reference intervals, biological variation, and diagnostic value in polycystic ovary syndrome, Clinica Chimica Acta (2015)
- Calculated free testosterone in men: comparison of four equations and with free androgen index, Annals of Clinical Biochemistry (2006)
- Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome, Journal of Clinical Endocrinology & Metabolism (2023)
- Sex steroids and sex steroid-binding globulin levels amongst middle-aged and elderly men and women from general population, European Journal of Clinical Investigation (2022)
- Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action, Journal of Endocrinology (2016)
AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.