Free Androgen Index

FAI at a glance

What is the Free Androgen Index (FAI)?

The free androgen index measures how much biologically active, or free, testosterone is circulating in the blood. It is not a hormone in its own right and it is not measured directly. It is worked out from two hormones that are measured: total testosterone and Sex Hormone Binding Globulin (SHBG).

Testosterone travels around the body in two states. Most of it is bound to proteins and cannot do anything. A small fraction is unbound and free to act on tissues such as skin, hair follicles, muscle and the ovaries. Total testosterone counts both states together, which is why two people with an identical total testosterone result can have very different symptoms. FAI is the number that separates them.

FAI is most useful in women, where it can pick up androgen excess that a total testosterone result on its own would miss.

Note: if you take high dose biotin therapy (more than 5mg a day), collect your FAI sample at least 8 hours after your last dose, as biotin can interfere with the assay.

What are androgens, and what do they do?

Androgens are the group of hormones responsible for male characteristics such as facial hair and a deeper voice. The best known androgen is testosterone.

Both sexes make androgens, just in very different quantities. In men they are produced mainly by the testes, in women mainly by the ovaries, and in both sexes the adrenal glands contribute a share. In women, androgens are not simply a male leftover. They support bone strength, muscle mass, mood, energy and sex drive, and they are the raw material the body converts into oestrogen.

Because androgens affect so many visible tissues, the symptoms of an imbalance tend to show up on the skin and hair long before anyone thinks to test for it. 

A useful way to picture what androgens govern is the following short list.

• Hair growth patterns, including scalp hair, facial hair and body hair
• Skin and oil production, which is why acne is a common sign of excess
• Muscle and bone, where androgens help maintain mass and density
• Libido, mood and energy, in women as well as men
• Ovarian function, where excess androgens can disrupt ovulation

Why SHBG determines your Free Androgen Index

SHBG is a protein made by the liver. Its job is to grip sex hormones and carry them through the bloodstream. While testosterone is held by SHBG it is effectively out of action.

This is why SHBG sits at the heart of the calculation. If your SHBG is high, more of your testosterone is locked up and your FAI falls. If your SHBG is low, more testosterone is left free and your FAI rises, even when your total testosterone has not changed at all.

Several everyday things move SHBG, which in turn moves FAI. The main ones are set out below.

• The combined pill and oral oestrogen, which push SHBG up sharply and pull FAI down
• Body weight and insulin resistance, which push SHBG down and pull FAI up
• Thyroid function, where an overactive thyroid raises SHBG and an underactive thyroid lowers it
• Liver health, since SHBG is made in the liver
• Age and menopause, which shift SHBG gradually over time

Illustration showing why SHBG determines your free androgen index, comparing high SHBG with more testosterone bound and low SHBG with more free testosterone.

Free Androgen Index vs free testosterone vs total testosterone

These three results are related but they are not interchangeable, and mixing them up is one of the most common reasons people misread their own report.

• Total testosterone is the full amount in your blood, bound and unbound together. It is measured directly.
• Free testosterone is the unbound fraction only. It can be measured directly by equilibrium dialysis, which is considered the most accurate method, or estimated using a calculation.
• Free androgen index is an estimate of that same free fraction, expressed as a ratio rather than a concentration. It is the quickest and cheapest of the three, which is why it appears on so many hormone panels.

Put simply, FAI is not the same thing as free testosterone. It is a shorthand for it, and like any shorthand it works well in some situations and less well in others. It performs best in women with a normal SHBG, and it becomes unreliable when SHBG sits at either extreme.

You may also see it written as serum free androgen index. This means exactly the same thing. Serum is simply the part of the blood sample the two underlying hormones are measured in.

How to calculate the Free Androgen Index

The FAI formula

The calculation is straightforward. You divide total testosterone by SHBG, then multiply by 100.

FAI = (total testosterone ÷ SHBG) × 100

Both values must be in the same units, which in the UK means nmol/L for each. If your report gives testosterone in ng/dL, it needs converting before the sum will work.

Diagram of the free androgen index formula, total testosterone divided by SHBG multiplied by 100, with a worked example.

FAI calculator

Rather than working it out by hand, you can enter your two results below and the calculator will do it for you, then tell you whether the answer sits inside or outside the reference range.

Remember that a calculator gives you a number, not a diagnosis. Any result that sits outside the range, or any result that comes alongside symptoms, is worth discussing with a doctor.

Why FAI is a ratio, not nmol/L

FAI is a ratio. You are dividing one nmol/L figure by another nmol/L figure, and the units cancel each other out. What is left is a plain number with no units at all. Because that number is then multiplied by 100, it is often written with a percent sign, which is why you will see a woman’s range quoted as 0.3 to 5.62.

So if you are looking for a free androgen index normal range in nmol/L, you will not find one, and that is not an oversight. Only the two inputs have units. The index itself does not.

Free Androgen Index normal range

The ranges below are the ones Forth applies when deciding whether a result is reported as low, healthy or high.

Reference ranges vary between laboratories, so always read your result against the range printed on your own report rather than one you have found elsewhere.

Normal FAI range in women

For women before menopause, Forth reports a healthy free androgen index as 0.3 to 5.62. After menopause the range narrows to 0.19 to 3.63, because androgen production falls with age. 

Forth customer data shows how this plays out in practice. Among women, the typical FAI is 2.52 in the 18 to 24 age bracket and falls steadily to 1.08 in women aged 60 and over.

Chart showing average free androgen index by age and sex, with women and men on separate scales.

The same pattern shows up when you group women by menopause status rather than age. Before menopause the typical FAI is 1.71. After menopause it is 1.18, roughly a third lower.

Normal FAI range in men

For men, Forth reports a healthy FAI as 24 to 104. Forth customer data shows a typical value of 55.4 in men aged 18 to 24, declining with each decade to 33.1 in men aged 60 and over.

There is an important caveat here that many sources leave out. UK guidance published through the Society for Endocrinology states that FAI is of limited value in men and might be considered only when total testosterone is not diagnostic of hypogonadism, particularly in men with obesity.

The reason is mathematical rather than clinical. The formula quietly assumes that there is far more SHBG binding capacity available than there is testosterone to fill it. That assumption holds reasonably well in women, whose testosterone levels are low. It does not hold in adult men, whose testosterone concentrations are high enough to saturate a meaningful share of the available SHBG. When researchers compared FAI against free testosterone measured by ultrafiltration, agreement was strong in women and much weaker in men.

In practice this means a man should treat FAI as supporting information rather than an answer. If you are investigating symptoms of low testosterone, a morning total testosterone result, repeated and interpreted alongside Luteinising Hormone (LH) and Follicle Stimulating Hormone (FSH), carries far more weight.

What do my results mean? Example FAI values interpreted

Numbers on their own are hard to place, so the table below shows what a handful of values would mean for different people. Use it as a guide to interpretation rather than a substitute for medical advice.

Context matters more than the number itself. Before drawing conclusions, check what else is going on.

• Are you taking hormonal contraception or HRT? Both change SHBG and therefore change FAI.
• Has anything changed with your weight, thyroid or liver? All three move SHBG.
• Do you have symptoms? A borderline number with no symptoms means something very different from the same number alongside hirsutism and irregular periods.
• Is this a single result? Hormones fluctuate, so one reading is a snapshot, not a verdict.

Reassuringly, most results are normal. Across all Forth FAI results, 81.7% came back within range. Among women, 86.4% were healthy, 10.4% were high and 3.3% were low.

Bar chart showing the percentage of free androgen index results that are low, healthy and high, split by sex.

High Free Androgen Index (raised FAI)

A raised free androgen index means more of your testosterone is biologically active than expected. In women this is the pattern behind most cases of clinical androgen excess. In Forth data, 10.4% of women’s results came back above range.

Symptoms and signs of high androgens in women

High androgen levels tend to announce themselves through the skin, hair and menstrual cycle. The most common symptoms of high androgen levels in females are listed below.

• Irregular periods, or no periods at all
• Excess hair growth on the face, chest, back or buttocks, known as hirsutism
• Thinning hair or hair loss in a male pattern at the temples and crown
• Persistent acne, often along the jawline, that does not respond to normal treatment
• Weight gain, particularly around the middle
• Difficulty getting pregnant

If you have been wondering why you are getting facial hair, or you have searched for a blood test for hair loss in females, FAI is usually part of the answer. It is worth asking your GP or requesting a hormone panel rather than treating the symptom cosmetically and leaving the cause unexamined. In more pronounced cases, androgen excess can produce secondary male characteristics such as a deepening voice, and this should always be assessed by a doctor.

What causes a raised FAI in women

The most common cause of high androgen levels in women is PMOS. PMOS affects roughly 10% of women and is characterised by overproduction of testosterone by the ovaries.

PMOS is associated with several other conditions, which is part of why identifying it matters.

• Infertility
• Obesity and metabolic syndrome
• Heart disease
• Depression
• Endometrial cancer

PMOS is not the only explanation, and the other causes of hyperandrogenism are set out below.

• Congenital adrenal hyperplasia, a group of inherited conditions affecting the adrenal glands that results in low cortisol and raised androgens
• Cushing’s disease, often caused by a benign pituitary tumour, which drives excess ACTH and in turn excess cortisol and androgens
• Adrenal tumours, which are rare but important to exclude
• Low SHBG, which raises FAI without any change in testosterone production at all

That final point deserves attention, because Forth data shows how strongly body weight feeds into it. Among women, the typical FAI is 1.33 in those with a BMI between 18.5 and 24.9, and 2.74 in those with a BMI of 30 or above. The number roughly doubles.

Chart showing median free androgen index by BMI group in women, rising with increasing BMI.

The mechanism is insulin. Higher insulin levels suppress SHBG production in the liver, which leaves more testosterone unbound and pushes FAI up. It also stimulates the ovaries to produce more testosterone, which is why weight and PMOS so often travel together.

FAI and PMOS: the diagnostic cut-off

Diagnosis in adults follows the 2023 international evidence-based guideline, which requires at least two of the following three features.

• Androgen excess, shown either by visible signs such as hirsutism, persistent acne or male pattern hair loss, or by a blood test such as FAI. Either route satisfies this criterion on its own.
• Ovulatory dysfunction, usually appearing as irregular, infrequent or absent periods.
• Polycystic ovary morphology, shown either by ultrasound or by a raised anti-Müllerian hormone (AMH) result.

That third point is the part that changed most recently, and it matters for anyone being assessed outside a gynaecology clinic. Until 2023 the only accepted route was a scan. The current guideline accepts AMH as an alternative, because AMH tracks closely with the follicle count an ultrasound would show and can be measured from a blood sample, which makes assessment possible in primary care rather than only where scanning is available. Whichever route is used, other causes of the same features have to be excluded first, which is why thyroid function and prolactin are usually checked alongside.

FAI speaks directly to the first of those three criteria. International guidance recommends calculated free testosterone, FAI or calculated bioavailable testosterone as the way to assess biochemical hyperandrogenism, because total testosterone alone is not sensitive enough.

There is a shortcut worth knowing about. If your FAI is high and your periods are irregular, you already meet two of the three criteria, so neither an ultrasound nor an AMH result is needed to make the diagnosis. Two caveats apply. These rules are for adults: in adolescents both androgen excess and ovulatory dysfunction are required, and neither ultrasound nor AMH should be used. The morphology criterion is also not recommended within 8 years of a first period, since a high follicle count is normal at that stage.

There is no single universal cut-off, and this is where a lot of online material overstates its case. Different laboratories and studies use different thresholds, and a value above 5 is commonly treated as suggestive of biochemical hyperandrogenism. Forth reports a premenopausal FAI above 5.62 as high. Either way, a raised FAI supports a PMOS diagnosis, it does not make one on its own.

A full PMOS workup normally includes several hormones alongside FAI.

• LH and FSH, since an altered ratio between them is common in PMOS
• Anti-Müllerian hormone, which is typically raised
• Prolactin and TSH, to rule out other causes of irregular cycles
• Progesterone, to confirm whether ovulation is happening
• HbA1c, to check for insulin resistance

Can high androgens affect fertility?

High androgen levels can interfere with ovulation, and if you are not ovulating regularly, conceiving becomes harder. This is one of the main routes by which PMOS affects fertility.

The encouraging part is that this is often modifiable. Because insulin resistance and body weight both push FAI upwards, changes that improve insulin sensitivity frequently improve ovulation as well. Fertility difficulties linked to androgen excess are also among the more treatable causes, so an abnormal FAI is a reason to seek advice rather than a reason to assume the worst.

High Free Androgen Index in men: causes and symptoms

It is uncommon for androgen levels to become genuinely too high in men, and when they do the cause is usually external rather than internal.

• Anabolic steroids are the most frequent explanation. They are a synthetic form of testosterone and have a powerful effect on the body, including increased muscle mass, which is why they are used to enhance sports performance.
• Certain cancers can also raise androgens. Research suggests men with higher free testosterone and the growth hormone IGF-I were more likely to be diagnosed with prostate cancer. Rarely, high androgen and oestrogen levels in men are linked to testicular cancer, specifically Leydig cell tumours.

The effects depend entirely on the cause. If the adrenal glands are responsible there may be few outward signs, though testicular function and fertility can still be affected. Where anabolic steroid use is the cause, the symptoms are more recognisable.

• Infertility and low sperm count
• Erectile dysfunction
• Shrunken testicles
• Hair loss
• Breast development
• Acne
• Stomach pain

Bear in mind the caveat from earlier. Because FAI is unreliable at the male end of the SHBG scale, a high FAI in a man should always be checked against a direct total testosterone result before anything is concluded.

Low Free Androgen Index

A low free androgen index means less testosterone is biologically available than expected. In Forth data, 3.3% of women’s results and 10.8% of men’s results came back below range.

Symptoms of low androgens in women

Low androgen levels in women are easy to miss, because the symptoms are diffuse and are often put down to stress, age or poor sleep. The most common androgen deficiency symptoms are set out below.

• Low sex drive, reduced arousal and less sexual enjoyment
• Reduced pubic hair
• Low bone mass
• Loss of muscle mass
• Reduced quality of life, including low mood and lack of energy
• More frequent vasomotor symptoms such as night sweats and hot flushes
• Insomnia
• Depression
• Headaches

What causes a low FAI in women

Age is the most common factor. Androgens decline naturally over time, and some women barely notice while others are very sensitive to the change. Before menopause, women produce around three times as much testosterone as oestrogen, so the fall matters more than people expect.

Medication is the next most common cause, and Forth data shows this clearly:

• Among women taking the combined pill or using a vaginal ring, the typical FAI is 0.66. 
• Among women taking no hormonal contraception the typical FAI is 1.73. 

That is a drop of roughly 60%, and it happens because oral oestrogen raises SHBG.

Progesterone-only methods behave very differently. Women using the progesterone-only pill, Mirena, coil or implant had a typical FAI of 1.85, slightly higher than women using nothing at all. This is the single most useful thing to know before you panic about a low result. If you are on the combined pill, a low FAI may simply be your contraception showing up in your bloodwork.

Chart comparing median free androgen index in women on hormonal contraception versus women not using it, split by contraceptive type.

Beyond age and medication, the other causes of low androgen levels in females are listed below.

• Surgery to remove one or both ovaries, known as oophorectomy
• Surgery to remove the adrenal gland, known as adrenalectomy
• Adrenal disease
• Pituitary disease
• Premature ovarian failure
• Turner’s syndrome
• HIV infection
• Medications, including high doses of corticosteroids and oestrogen preparations such as the pill or HRT

Low Free Androgen Index in men

A low free androgen index in men can indicate testosterone deficiency. Age associated testosterone deficiency syndrome, also called late onset hypogonadism, is diagnosed when blood testosterone sits below the reference range for young healthy adult men.

Low testosterone is also commonly associated with type 2 diabetes. Some research suggests higher testosterone levels reduce the risk of type 2 diabetes in men, making testosterone a protective factor against metabolic disease.

Common symptoms of late onset hypogonadism are set out below.

• Depression
• Difficulty concentrating
• Hot flushes
• Fatigue
• Reduced muscle mass and muscle weakness
• Hair loss
• Low sex drive

In Forth data, 24.5% of men’s FAI results fell outside the reference range, split between 10.8% below and 13.7% above. Given the known limitations of FAI in men, a large share of these will reflect the calculation rather than a genuine hormone problem, which is exactly why total testosterone should always be read alongside it.

Can you increase a low Free Androgen Index?

The right approach depends entirely on why it is low in the first place. 

• If the cause is your contraception, the number will usually recover if you change method, and that is a conversation to have with your GP or sexual health clinic rather than something to act on alone. 
• If the cause is a very high SHBG driven by thyroid overactivity or liver issues, treating the underlying condition is what moves the number.

Where there is no specific medical cause, the general levers are the ones you would expect.

• Resistance training, which supports both testosterone production and muscle mass
• Adequate sleep, since testosterone is produced largely during sleep and short sleep measurably lowers it
• Enough dietary fat and protein, because very low fat diets are associated with lower testosterone
• Managing chronic stress, as sustained high cortisol suppresses sex hormone production
• Correcting deficiencies, particularly vitamin D, zinc and magnesium

Testosterone therapy for women is a specialist area with limited licensed options in the UK, so it should only be considered under medical supervision.

What to do if your FAI is abnormal

How to lower a high Free Androgen Index

Because FAI is a ratio, there are two ways to bring it down: reduce testosterone production, or raise SHBG so that more testosterone is bound. In practice, most of what works does both.

The most effective lever is metabolic. Given the BMI pattern in Forth data, improving insulin sensitivity is the intervention with the strongest evidence behind it and the largest effect. That means the following.

• Reducing excess weight where relevant, since even modest loss can raise SHBG and restore ovulation
• Cutting refined carbohydrates, which lowers the insulin spikes that suppress SHBG
• Regular exercise, particularly a mix of resistance and aerobic work, which improves insulin sensitivity independently of weight

Forth data also shows a modest association with exercise in women. Typical FAI was 1.63 among women exercising up to 5 hours a week and 1.30 among those exercising 11 to 15 hours a week. This is a smaller and less consistent effect than the BMI pattern, so treat it as a hint rather than a rule.

Several foods have been suggested to help lower testosterone levels in women, although the scientific evidence supporting them is limited.

• Vegetable oils and oily fish: increased omega-3 polyunsaturated fatty acid intake has been positively linked with testicular volume in young men. Omega-3s are found in oily fish, soybean oil, flaxseeds and walnuts.
• Spearmint tea: some studies suggest spearmint tea has anti-androgenic effects that could benefit women with PMOS. Animal studies have shown that a combination of flaxseed and spearmint extract could reduce testosterone, increase oestrogen and progesterone, and may help regulate menstrual cycles in PMOS.
• Red reishi mushrooms: used for centuries in Asian medicine, red reishi lowers 5-alpha reductase, the enzyme that converts testosterone into the more potent dihydrotestosterone (DHT), so it may have potential to lower testosterone levels.
• Liquorice root: a common ingredient in sweets and drinks, consuming 7g of liquorice root daily reduced testosterone by a quarter in men after a week, with similar results demonstrated in women.

Where lifestyle is not enough, medical options exist: 

• The combined pill, often prescribed to manage hirsutism and acne precisely because it raises SHBG and lowers FAI. 
• Anti-androgen medications such as spironolactone and cyproterone acetate, and insulin sensitising drugs such as metformin. 

All of these are prescription only and need to be discussed with a doctor, since they carry their own considerations and are not suitable if you are trying to conceive.

Does vitamin D reduce androgens?

Some research has reported a significant benefit of vitamin D supplementation on male hormone concentrations. A study published in 2021 found that vitamin D supplementation lowered hirsutism scores and androgen levels in overweight women with PMOS.

This matters more than it might sound, because deficiency is widespread. Forth data shows that 57% of UK adults are deficient in vitamin D. The NHS recommends that all UK adults consider a daily 10 microgram vitamin D supplement during autumn and winter, because the sun is not strong enough for the body to make it.

Vitamin D is not a treatment for androgen excess on its own. Correcting a deficiency is sensible regardless, and it may help at the margins.

When to see a doctor

A blood test tells you a number. A doctor tells you what to do about it. Book an appointment if any of the following apply.

• Your periods have become irregular, very infrequent or have stopped
• You have noticed new or worsening facial or body hair growth
• You are losing scalp hair in a male pattern
• You have been trying to conceive for 12 months without success, or 6 months if you are over 35
• Your symptoms have come on quickly or are severe, which needs prompt assessment
• You are a man with symptoms of low testosterone and a low FAI result

Bring your results with you, ideally including total testosterone and SHBG rather than the FAI figure alone, since your GP will want to see the inputs as well as the output.

How to test your androgen levels: the FAI blood test

If you have been searching for how to check androgen levels in females, or which blood test shows androgen levels, this is the section you need.

What the FAI blood test measures

There is no such thing as a machine that measures FAI. An FAI blood test measures two things and calculates the third.

• Total testosterone, measured directly from your blood sample
• SHBG, measured directly from the same sample
• FAI, calculated from the two and reported alongside them

This is why the FAI cannot be ordered in isolation. Any test that reports FAI will always include the two underlying results, which is useful, because you need all three to interpret any of them properly.

How to prepare: timing, cycle day and fasting

Preparation is simpler than most people assume. The key points are below.

• Fasting is not required. You can eat and drink normally beforehand.
• Collect in the morning where possible. Testosterone follows a daily rhythm and peaks in the morning, so a morning sample is more representative and more comparable if you retest later.
• Cycle timing matters less than you would expect. Forth data covering women with regular cycles and no hormonal contraception shows a typical FAI of 1.69 on days 1 to 5, 1.78 on days 6 to 14 and 1.62 on days 15 to 28. The differences are small.
• Pause high dose biotin. If you take more than 5mg a day, collect your sample at least 8 hours after your last dose.
• Note down your contraception and any HRT. Given how strongly these move the result, your report is far easier to interpret if you record them.

Chart showing median free androgen index by day of the menstrual cycle in women with regular periods.

If you are testing other hormones at the same time, cycle day may still matter a great deal. Progesterone in particular needs to be collected around day 21 of a 28 day cycle, so follow the guidance for the full panel rather than for FAI alone.

Understanding your results and turnaround times

Results are usually available within 2 working days of your sample reaching the lab, and you will be notified as soon as they are ready to view.  Your report will show your FAI alongside total testosterone and SHBG, each with its reference range, and will flag anything outside that range. 

Read the three together rather than separately. A high FAI with a normal testosterone and a low SHBG tells a different story from a high FAI driven by high testosterone, and only the first is really a binding problem rather than a production one.

Forth blood tests that include the Free Androgen Index

All of the tests below report the free androgen index. Choose the one that matches what you are trying to find out.

• Female Hormone Imbalance Test: a single day snapshot of your key female hormones alongside thyroid function, covering 11 biomarkers. This is the right starting point if you have symptoms such as irregular periods, acne or unwanted hair growth and want to understand your overall hormone picture.
• PMOS Test, built specifically around the biochemical side of a PMOS assessment, bringing together androgens with the other hormones used to support or rule out a diagnosis. Choose this if PMOS has been raised as a possibility, either by you or by your GP.
• Advanced Blood Test: a broad health check covering hormones alongside kidney function, liver health, inflammation, cholesterol, vitamins and minerals. This is the most common route for men, since it places FAI in the context of total testosterone, SHBG, FSH, LH and prolactin rather than in isolation.
• Ultimate Blood Test: the most comprehensive panel Forth offers, for anyone who wants the fullest possible picture in one go rather than testing area by area.

If you only want to check specific markers, you can hand pick them in the bespoke builder. Selecting the free androgen index automatically includes the total testosterone and SHBG it is calculated from, so you will see all three on your report. This is the most economical option if you have tested before and simply want to recheck.

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