Free Androgen Index (FAI): What High & Low Levels Mean

1. What is the free androgen index?

Most of the testosterone in your blood is not free to do anything. It travels bound to a carrier protein called SHBG (sex hormone-binding globulin) and, more loosely, to albumin. Only a small unbound fraction is biologically active, the part that can actually enter cells and produce an effect. Total testosterone counts all of it, bound and unbound together, which is why a “normal” total can hide a meaningful problem.

The free androgen index is a quick estimate of that active fraction. It compares how much testosterone you have to how much SHBG is available to mop it up. A high index means more testosterone is going spare relative to its carrier, so more is biologically active. A low index means most of it is locked away.

Its main clinical job is investigating hyperandrogenism in women: the cluster of signs, from acne and unwanted hair growth to irregular periods, that points towards conditions such as polycystic ovary syndrome (PCOS). It is a recommended baseline investigation when raised androgens are suspected, and it is one of the markers included in the Hormone Balance panel.

2. Free androgen index normal range (UK)

FAI is reported as a percentage. The ranges below reflect a representative UK NHS laboratory (Gloucestershire Hospitals NHS pathology). Note that FAI ranges are not standardised nationally, so always read your result against the range printed on your own report.

The reason men barely feature here is scale. Men have far more testosterone and proportionally less SHBG, so a male FAI runs many times higher than a female one and the index loses its diagnostic value. In men, the better measure of the active fraction is a calculated free testosterone, which the British Society for Sexual Medicine recommends when a total testosterone result sits in the grey zone.

3. How FAI is calculated (and the SHBG trap)

The formula is simple:

Because SHBG sits on the bottom of that fraction, it does most of the work. If SHBG drops, the index climbs even when your actual testosterone has not moved at all. A woman with a total testosterone of 1.5 nmol/L and an SHBG of 60 has an FAI of 2.5. The same testosterone with an SHBG of 20 gives an FAI of 7.5, three times higher, from a change in the carrier protein alone.

This is the SHBG trap, and it cuts both ways. It is exactly why FAI is informative in PCOS, where insulin resistance pushes SHBG down and drives the index up. But it is also why FAI becomes unreliable when SHBG is very low. The 2025 systematic review that informs current PCOS guidance notes that FAI can be inaccurate when SHBG falls below 30 nmol/L, because the calculation leans so heavily on a number that is itself disturbed.

The practical takeaway: an FAI is only as trustworthy as the SHBG underneath it. Always read the two together. A high FAI driven by a low SHBG tells a different story from a high FAI driven by genuinely high testosterone, and the distinction changes what comes next.

4. High FAI: what it means

A raised FAI means more of your testosterone is biologically active than the carrier proteins can contain. In women, the dominant cause is polycystic ovary syndrome, which affects roughly 1 in 10 women in the UK. PCOS combines two pressures on the index: ovaries that produce a little more testosterone, and insulin resistance that suppresses SHBG, lifting the FAI from both ends at once.

The clinical signs that often accompany a high FAI include:

• Hirsutism, coarse hair on the face, chest or back
• Persistent or adult-onset acne
• Scalp hair thinning in a male pattern
• Irregular, infrequent or absent periods
• Difficulty conceiving

Less common causes of a high FAI include congenital adrenal hyperplasia, certain ovarian or adrenal tumours, Cushing's syndrome, and the use of anabolic steroids or testosterone therapy. A markedly raised testosterone alongside a high FAI, or a rapid onset of symptoms, is a reason for prompt clinical assessment rather than watchful waiting.

A high FAI on its own is not a diagnosis. It is biochemical evidence of hyperandrogenism that a qualified clinician interprets alongside your symptoms, your LH and FSH pattern, and, where relevant, an ovarian ultrasound. The diagnostic criteria for PCOS require more than one number.

5. Low FAI: what it means

A low FAI means very little of your testosterone is in its active form. In most women this is unremarkable, because female testosterone is low to begin with. It becomes relevant when it sits alongside symptoms such as persistently low libido, low energy or low mood, particularly around and after menopause when ovarian output falls.

The usual driver of a low FAI is a high SHBG, which locks up whatever testosterone is present. Causes of high SHBG include ageing, an overactive thyroid, oestrogen therapy including the combined contraceptive pill, and liver disease. This is why the same caution applies in reverse: a low FAI from a high SHBG is a different situation from a low FAI from genuinely low testosterone production.

Testosterone has a recognised role in women's energy, mood and sexual desire, and testosterone testing in women is often overlooked. If your FAI is low and you have symptoms, the conversation is best had with a clinician who can weigh the whole picture, including whether a high SHBG is the real story.

6. FAI vs free testosterone vs total: which is best?

There are three common ways to assess androgens in women, and they measure slightly different things:

• Total testosterone, everything in the blood, bound and unbound
• Free androgen index, an estimate of the active fraction based on the testosterone to SHBG ratio
• Calculated free testosterone, a more complete estimate of the active fraction that also factors in albumin

A 2025 systematic review and meta-analysis assessed how well each performs for detecting biochemical hyperandrogenism in PCOS. The headline finding is that they perform broadly similarly as first-line tests, with areas under the curve of around 0.85 to 0.87. Total testosterone and free testosterone were recommended as the first-line laboratory tests, with free testosterone calculated by FAI accepted as a practical route.

In plain terms: FAI is a good, cheap, widely available screen, and for most women it does the job. Its weak spot is the low-SHBG scenario described above, where a calculated free testosterone gives a steadier reading. A sensible panel reports total testosterone, SHBG and FAI together, so the three can be cross-checked rather than any single number being read in isolation.

7. What raises and lowers SHBG (and therefore FAI)

Since SHBG sits at the heart of the calculation, anything that moves it moves your FAI. This matters when you interpret a result, because a “hormone problem” can sometimes be a binding-protein problem in disguise.

The insulin link is the one to remember in PCOS. Improving insulin sensitivity, through weight loss where relevant, regular movement and reducing refined carbohydrate, tends to raise SHBG and lower FAI. That is also why tracking insulin resistance markers alongside your androgens gives a far more actionable picture than the hormone numbers alone.

8. When and how to test for an accurate result

Because FAI depends on both testosterone and SHBG, timing and technique matter as much as for any hormone test:

• Morning sample. Testosterone is highest in the morning, so a sample before 11am gives the most representative reading.
• Cycle timing. If you have regular periods, days 1 to 5 of the cycle are the conventional window for an androgen workup. Irregular cycles, common in PCOS, make timing less critical.
• Off the combined pill where possible. Oestrogen raises SHBG and suppresses ovarian testosterone, which distorts both the index and the diagnosis. A clinician can advise on whether and how long to pause before testing.
• Test the trio together. Total testosterone, SHBG and FAI from one sample, so the numbers are internally consistent and the SHBG can be read in context.

The Hormone Balance panel measures testosterone, SHBG and the free androgen index alongside FSH and LH, the combination used to investigate PCOS and the perimenopausal transition. It is a home finger-prick kit analysed by UKAS-accredited UK laboratories, and the report shows each value against its reference range. For a fuller picture of the carrier protein that drives your index, see our dedicated SHBG guide.

Frequently asked questions