FAI (Free Androgen Index): Meaning and Value

What is FAI (free androgen index)?

Testosterone travels in your blood mostly clamped to SHBG, a protein your liver makes, and for a smaller part loosely to albumin. Only a few percent is truly free, and only that free part can enter the cells and works. How much is free therefore depends on two things: how much testosterone there is, and how much SHBG there is to bind it. Measuring free testosterone directly is difficult and unreliable; the free androgen index bypasses that by dividing total testosterone by SHBG, both in the same unit from the same tube, and giving the result as a percentage. That makes the index sensitive to exactly what is often going on in women. Belly fat and insulin resistance make the liver produce less SHBG, and then the free part rises while total testosterone stays the same. The pill does the reverse: oestrogen makes the liver produce more SHBG, and the free part falls. A woman with a normal total testosterone and a low SHBG can therefore have symptoms of too much male hormone, and the index shows that where the total misses it. The result is given as a percentage. The only published band runs for a man of 20 to 50 years from 24.3 to 110.2 per cent, for a woman of 20 to 46 years from 0.65 to 10.93 per cent and for a woman from 47 years from 0.23 to 6.8 per cent. The difference between men and women follows from the formula: men have a multiple of the testosterone and less SHBG.

Why is FAI (free androgen index) relevant?

The index is above all a women's value. With excess hair growth on face, chest or belly, acne after puberty, hair loss on the crown or a cycle that is irregular or absent, the question is whether too much male hormone is working, and total testosterone alone often does not answer that question: it sits within the band in half of women with PCOS. The index picks up what the total misses, because the low SHBG that belongs with PCOS pushes the numerator and the denominator the wrong way at the same time. In men the index is less useful, because the own bands of total testosterone and free testosterone usually say enough there. The second reason is that the index shows where a deviation comes from. A high index with a normal testosterone is a low SHBG, and that is almost always belly fat and insulin resistance, or an underactive thyroid; a high index with a high testosterone is the ovary or the adrenal gland making too much. A low index with a normal testosterone is a high SHBG, from the pill, an overactive thyroid, eating little or getting older. That direction decides what you do about it. The honest limitation is that the index is an estimate that fits less well with a very high or very low SHBG, and that only one Dutch table exists for it. Anyone reading the index therefore always reads the two numbers it comes from alongside.

FAI (free androgen index) reference ranges

What counts beside the number:

The total testosterone and the SHBG the index comes from, because the ratio does not say which of the two moved
Your sex and age, because the band for men sits more than ten times higher than for women and falls in women after the menopause
Whether you take the pill or are pregnant, because oestrogen lifts SHBG and lowers the index
Your waist, and glucose and insulin from the same draw, because insulin resistance is the most common reason for a low SHBG
Your thyroid, because an underactive thyroid lowers SHBG and an overactive one lifts it
In a woman with symptoms: DHEA-S, LH and FSH, and with a testosterone far above the band how fast it rose

FAI (free androgen index) reference ranges
Group	Reference range
Men 20 to 50 yrsThe own band of the only lab publishing a reference value for the index; in men total testosterone usually says enough	24.3–110.2 %
Women 20 to 46 yrsMore than ten times lower than the men's band, because women have a fraction of the testosterone and more SHBG; the band that counts with symptoms of too much male hormone	0.65–10.93 %
Women from 47 yrsLower after the menopause, because the ovaries make less testosterone and SHBG rises with the years	0.23–6.8 %

There is no national reference value for the free androgen index, and only one Dutch table exists, without a men's band above fifty; if your lab reports a different range, hold to that. The index is a calculation from total testosterone and SHBG, both from the same tube, and without those two numbers it cannot be read: the ratio does not say which of the two moved.

FAI = total testosterone / SHBG, expressed as a percentage

Both inputs come from the same tube of blood and are stated in nmol/L, so the division yields a ratio. Clinical Diagnostics reports that ratio as a percentage, and the bands below are in that same unit.

Total testosteronenmol/LSHBGnmol/L

÷ × 100 =

What Dutch labs actually use

One lab publishes a reference value for the free androgen index: Clinical Diagnostics. Unilabs, Star-shl and Certe do not carry the index as a test with reference values of its own, so there is nothing to compare here. What the table does show is the difference between men and women: for a man of 20 to 50 years the index runs from 24.3 to 110.2 per cent, for a woman of 20 to 46 years from 0.65 to 10.93 per cent, and that follows from the formula, because men have a multiple of the testosterone and less SHBG.

Age

Woman aged 35 · only Clinical Diagnostics publishes a range

0.65 – 10.93%

At Clinical Diagnostics this is normal.

Clinical Diagnostics

0.65–10.93 %

Every age, per lab

What Dutch labs actually use
Laboratory	Group	Reference range
Clinical Diagnostics2026	Men 20 to 50 yrs	24.3–110.2 %
	Women 20 to 46 yrs	0.65–10.93 %
	Women from 47 yrs (post-menopausal)	0.23–6.8 %

Each band as the lab publishes it, retrieved in 2026, unless marked otherwise.

FAI (free androgen index) high or low: what it means

A low index means little testosterone is free, from a low total or from a high SHBG. A low total testosterone in women comes from age, the pill or eating too little, and in men from age, belly fat, lack of sleep or testicles that make less. A high SHBG comes from oestrogen, so the pill or a pregnancy, from an overactive thyroid, from eating little or a low weight, and from getting older. In a woman a low index is rarely a problem; in a man with symptoms of little energy, little desire and less muscle it is a reason to look at total and free testosterone themselves. A high index means a lot of testosterone is free, from a high total or from a low SHBG. In a woman with excess hair growth, acne or an irregular cycle a high index with a normal or slightly raised testosterone is usually PCOS, in which belly fat and insulin resistance lower SHBG. A high index with a testosterone far above the band, more than double, is rare and points to an ovary or adrenal gland making too much on its own, and that is investigated; DHEA-S then says whether it is the adrenal gland. In a man a high index is usually a low SHBG from belly fat, or testosterone from outside. What you do about it: with a high index from a low SHBG, weight, exercise and a diet with few fast sugars are the levers, because less belly fat and more insulin sensitivity make the liver produce more SHBG again and the index fall. In PCOS the pill or drugs that block the hormone treat the symptoms of excess hair growth and acne. With a low index from the pill nothing is wrong. With a low index in a man, sleep, weight and strength training are the first levers, and with a genuinely low testosterone with symptoms a doctor decides on topping up. Read the index beside the total testosterone and the SHBG it comes from, because the ratio does not say which of the two moved, and beside free testosterone if the lab measures it. In a woman with symptoms DHEA-S, LH and FSH belong with it, and in everyone glucose and insulin, because insulin resistance is the most common reason for a low SHBG.

What lowers it

Little testosterone free, from a low total or from a high SHBG.

What lowers it: FAI (free androgen index)
Cause	How often
A low total testosteroneIn women from age, the pill or eating too little; in men from age, belly fat, lack of sleep or testicles that make less	Often
A high SHBGOestrogen from the pill or a pregnancy, an overactive thyroid, eating little or a low weight, and getting older make the liver produce more SHBG	Often

What raises it

A lot of testosterone free, from a high total or from a low SHBG.

What raises it: FAI (free androgen index)
Cause	How often
A high total testosteroneIn a woman PCOS, or rarely an ovary or adrenal gland making too much on its own; in a man testosterone from outside	Often
A low SHBGBelly fat and insulin resistance make the liver produce less SHBG, as does an underactive thyroid; the free part rises while the total stays the same	Often

How does a FAI (free androgen index) blood test work?

Referral: Not needed, and there is no separate test: the index is a calculation from total testosterone and SHBG, which you can have measured yourself and make with the calculator on this page.
Fasting: Preferably yes, because a meal lowers testosterone for a few hours, and glucose and insulin from the same draw need fasting.
When: In the morning between 07.00 and 10.00, because testosterone peaks in the morning, after a normal night and without a hard training session the day before. In a woman with a cycle in the first week after the period.
The draw: One tube of blood from a vein in your arm. Total testosterone and SHBG come from the same tube, and DHEA-S, LH, FSH, glucose and insulin can be added.
Repeating: With a deviating index after six to twelve weeks under the same conditions, because testosterone fluctuates from day to day. Anyone working on weight and insulin sensitivity measures again after three to six months.

Where to draw: You can draw at 250+ locations near you, no referral needed. See the locations

Frequently asked questions

What is the FAI?

The free androgen index: your total testosterone divided by your SHBG, the protein that binds testosterone, expressed as a percentage. Most of the testosterone in your blood is bound to SHBG and does nothing; only the free part works. Because measuring free testosterone directly is difficult, the index estimates it from those two numbers. It is a calculation, not a separate test.

What is a normal FAI value?

According to the only lab that publishes a band, for a man of 20 to 50 years 24.3 to 110.2 per cent, for a woman of 20 to 46 years 0.65 to 10.93 per cent and for a woman from 47 years 0.23 to 6.8 per cent. The difference between men and women follows from the formula. More important than the band are the two numbers the index comes from, because those say which way it is being pulled.

What does a high FAI mean?

That a lot of testosterone is free, from a high total or from a low SHBG. In a woman with excess hair growth, acne or an irregular cycle a high index with a normal or slightly raised testosterone is usually PCOS, in which belly fat and insulin resistance lower SHBG. A testosterone far above the band, more than double, is rare and points to an ovary or adrenal gland making too much. In a man it is usually a low SHBG from belly fat, or testosterone from outside.

What does a low FAI mean?

That little testosterone is free, from a low total or from a high SHBG. A high SHBG comes from the pill, a pregnancy, an overactive thyroid, eating little or getting older, and a low total in women from age or the pill and in men from age, belly fat or lack of sleep. In a woman a low index is rarely a problem; in a man with symptoms of little energy and less muscle you read total and free testosterone themselves.

Why do men and women differ so much?

Because both inputs work in the same direction. Men have a multiple of women's testosterone and at the same time less SHBG, so the numerator is larger and the denominator smaller. That is why the ceiling for men sits more than ten times higher and the floor almost forty times. That is not a deviation but biology, and it is the reason the index can already show something in a woman with a normal testosterone.

Why is there only one lab in the table?

Because only one lab publishes a reference value for the index; the other three do not carry the index as a test with a band of its own, though you can calculate it yourself from their testosterone and SHBG. One table is not a consensus, and the lab publishes no men's band above fifty. If your lab reports an FAI with a different range, hold to that range.

What is the difference from free testosterone?

Free testosterone is the value the index tries to estimate. Measuring it directly is difficult and unreliable at many labs, and that is why they calculate it, with the index or with a formula that also includes albumin. If your lab calculates or measures free testosterone itself, that says the same as the index without a calculation. With a very high or very low SHBG the index fits less well, and then the calculation with albumin is better.

How do I bring a high FAI down?

By letting SHBG rise again, and you do that with weight, exercise and a diet with few fast sugars: less belly fat and more insulin sensitivity make the liver produce more SHBG again, and the index falls along. In PCOS the pill or drugs that block the hormone treat the symptoms of excess hair growth and acne. Count on three to six months before the index moves, and then measure again under the same conditions.

Which tests include FAI (free androgen index)?

FAI (free androgen index) is in the Hormones men test (€129, also as an add-on for €109).

Related biomarkers

SHBGThe denominator; belly fat and insulin resistance lower it, the pill and an overactive thyroid lift it, and that shifts the index.Total testosteroneThe numerator of the index; in a woman with symptoms it sits within the band in half while the index is already raised.InsulinInsulin is the hormone your pancreas uses to move glucose out of your blood and into your cells. A fasting insulin shows how much effort your body puts in to keep your blood sugar normal, and that number moves years before the blood sugar itself does.DHEA-SDHEA-S is the storage form of DHEA, the hormone your adrenal glands make as raw material for testosterone and oestrogen. It peaks around your twenties and falls steadily after that, and because it does not move with the clock, it is the calm counterpart of cortisol from the same glands.Free testosteroneThe value the index tries to estimate; if the lab measures it, it says the same without a calculation.FSHFSH, in full follicle-stimulating hormone, is the signal with which your brain sets the ovaries or the testes to work: low when they respond well, high when they respond less, and permanently high after the menopause.

Read on

PCOSPCOS, polycystic ovary syndrome, is a hormonal disturbance in which the ovaries make too many male hormones (androgens). As a result ovulation often fails, and that shows in an irregular or absent period, excess hair, acne or hair loss. Insulin plays a leading role in it, and that is why losing weight with excess weight is the treatment that improves the whole picture. About 6% of women of childbearing age have PCOS.Low testosteroneA testosterone deficiency means the body makes less testosterone than it needs, and that complaints belong with it. Testosterone falls gradually in healthy men from 35 years, and that fall in itself is not a deficiency; belly fat, lack of sleep and poorly controlled diabetes lower it harder than age does. A deficiency is also called hypogonadism, or in older men late-onset hypogonadism or andropause.

Sources

1.Clinical Diagnostics, lab guide (Dutch): free testosterone (free androgen index). 2026. clinicaldiagnostics.nl

Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.

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