Free androgen index normal range: how to calculate and read FAI

Quick answer: the free androgen index normal range depends on sex and age. The London Health Sciences Centre (LHSC) laboratory lists 0.3 to 5.6 for women aged 20 to 49, 0.2 to 3.6 for women 50 and over, 35.0 to 92.6 for men aged 20 to 49 and 24.3 to 72.1 for men 50 and over. FAI = total testosterone (nmol/L) / SHBG (nmol/L) x 100. The 2023 international PCOS guideline accepts FAI as one way to assess high androgens in women.

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Free androgen index normal range

FAI is a ratio, so it has no unit, though some labs print it as a percentage. Ranges differ between labs and testosterone assays, so compare your result with the range on your own report.

How to calculate FAI

FAI = total testosterone (nmol/L) / SHBG (nmol/L) x 100. US labs usually report testosterone in ng/dL. Divide ng/dL by 28.84 to get nmol/L (testosterone's molar mass is 288.4 g/mol). SHBG is normally already in nmol/L.

Example: total testosterone 40 ng/dL is 40 / 28.84 = 1.39 nmol/L. With an SHBG of 50 nmol/L, FAI = 1.39 / 50 x 100 = 2.8.

The logic: most testosterone is bound tightly to SHBG and is not available to tissues. When SHBG is low, more testosterone is free, so the same total testosterone has a bigger effect. FAI captures that in one number. It is a rough index: labs that calculate free testosterone with the Vermeulen equation also account for albumin.

FAI in women and PCOS

For women, FAI is most useful when checking for high androgens, one of the three features used to diagnose polycystic ovary syndrome. The 2023 international PCOS guideline recommends calculated free testosterone, FAI or calculated bioavailable testosterone to assess biochemical hyperandrogenism, because direct assays of total testosterone are not precise enough at the low levels women have. A high FAI with irregular periods, acne or excess hair growth is a reason to see a doctor; a diagnosis needs symptoms, other tests and often an ultrasound or AMH. See our PCOS blood test guide.

SHBG matters as much as testosterone here: a low SHBG pushes FAI up even when total testosterone is normal, as the worked example below shows.

FAI in men

In men, FAI follows the same formula, and labs usually report calculated free testosterone alongside it, which accounts for albumin as well as SHBG. FAI is most informative when SHBG is unusually high or low, since that is when total testosterone alone misleads. Note that the LHSC range for men 50 and over starts lower, at 24.3.

Worked examples

Woman, low SHBG: total testosterone 45 ng/dL (1.56 nmol/L), SHBG 25 nmol/L. FAI = 1.56 / 25 x 100 = 6.2, above the 0.3 to 5.6 range for her age. With the same testosterone and an SHBG of 70 nmol/L, FAI would be 2.2, well inside the range. The difference is entirely SHBG.

Man, high SHBG: total testosterone 500 ng/dL (17.3 nmol/L), which looks fine. With SHBG 35 nmol/L, FAI is 49.5, inside 35.0 to 92.6. With SHBG 70 nmol/L, FAI is 24.8, under the range, which suggests less available testosterone than the total implies. Calculated free testosterone and symptoms decide what happens next, with a doctor.

Which test to use

How to prepare for the test

• Collect in the morning. MedlinePlus says testosterone is usually highest between 7 and 10 am, which is when labs draw it.
• Note your cycle day. For women with periods, the cycle day helps interpret the estradiol, progesterone, LH and FSH on the same panel.
• List medicines and supplements. MedlinePlus says you may need to stop certain medicines before a testosterone test, so tell your provider everything you take, but do not stop anything on your own.
• Compare with the same lab. FAI ranges depend on the testosterone and SHBG assays used.

At-home tests that report FAI

The free androgen index is on two SiPhox panels: Hormone Focus and Ultimate 360. SiPhox's Hormone Focus page lists both a female free androgen index and a free androgen index. Both panels also report total testosterone, calculated free testosterone, % free testosterone and SHBG, so you can check the arithmetic yourself.

For women who still have periods, note your cycle day when you collect, since it helps interpret the other hormones on the panel. SiPhox is a wellness service with no doctor review included. See our at-home hormone test for women and at-home testosterone test guides.

A note on home samples: SiPhox's partner lab tests capillary blood collected from the upper arm. SiPhox says its results show a 95% to 99% correlation with venous draws in its own quarterly comparisons against Labcorp and Quest, but it has not published a study of the EasyDraw device. If a result would change a treatment decision, confirm it with a venous test through your doctor.

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Sex hormones for men and women, including PSA for men and progesterone for women.

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$249 + $25 shipping per test

62 biomarkers per test

Everything on the four core panels plus iron studies, B12 and folate, GGT and PSA.

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Test androgens and SHBG at homeHormone Focus: 14 biomarkers per test, including FAI, testosterone, SHBG, DHEA-S, LH, FSH and prolactin.

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