Free Androgen Index: What Your Result Means for Hair
The free androgen index is a number worked out from two blood results, and it often appears on a hormone panel ordered when a woman has thinning hair, unwanted facial hair or irregular periods. This guide explains how the number is calculated, what NICE guidance regards as normal, why it can mislead, how it links to hair, and what to ask your GP about your own result.
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The free androgen index (FAI) estimates how much active testosterone is circulating in a woman's blood. NICE Clinical Knowledge Summaries describe it as 100 multiplied by total testosterone divided by sex hormone-binding globulin (SHBG), with a normal range usually below 5, though laboratories vary. A raised result points towards excess androgens, as seen in polycystic ovary syndrome, which the NHS now calls PMOS.
Key takeaways
- FAI = 100 x total testosterone divided by SHBG, according to NICE CKS.
- NICE CKS says the normal range is usually defined as lower than 5, but it varies between laboratories.
- Results are not reliable on hormonal contraception, because it changes SHBG, so tell your GP what you take.
- DermNet says most women with female pattern hair loss have normal androgen levels, so a normal result does not rule it out.
- The NHS lists excess hair growth and hair loss among the main symptoms of PMOS, previously called PCOS.
What is the free androgen index?
The FAI is a calculated ratio, not a separate blood test, that estimates how much testosterone in the blood is free to act on the body. Most testosterone travels bound to a carrier protein called sex hormone-binding globulin, and the bound portion is largely inactive, so the ratio adjusts total testosterone for how much carrier is available.
Androgens are the group of hormones that includes testosterone. Women make them in smaller amounts than men, in the ovaries and the adrenal glands, and they influence body hair, oil production in the skin and scalp hair. When there is more active androgen than usual, the effects tend to show up on the skin and hair first, which is why the ratio is often requested when someone mentions new chin hair, acne or a widening parting.
The number is mainly used for women. In men, total testosterone is much higher and the ratio behaves differently, so labs and clinicians usually rely on other measures. If you want the background on the carrier protein itself, our guide to SHBG and hair thinning explains what raises and lowers it.
How is the free androgen index calculated?
The calculation, as set out by NICE Clinical Knowledge Summaries, is 100 multiplied by the total testosterone result divided by the SHBG result. Both come from the same blood sample, testosterone in nanomoles per litre and SHBG in nanomoles per litre, so the units cancel out and the answer is a plain number.
| Total testosterone (nmol/L) | SHBG (nmol/L) | Calculation | Result |
|---|---|---|---|
| 1.5 | 60 | 100 x 1.5 / 60 | 2.5 |
| 1.5 | 30 | 100 x 1.5 / 30 | 5.0 |
| 2.5 | 25 | 100 x 2.5 / 25 | 10.0 |
These are worked examples to show the arithmetic, not reference values for your own result. They show something useful, though: the same testosterone level can give a very different index if the carrier protein changes. In the second row, testosterone has not moved at all, but halving SHBG doubles the ratio. That is why low SHBG on its own can push the index up.
What is a normal free androgen index result?
A normal result, according to NICE Clinical Knowledge Summaries, is usually defined as lower than 5, but NICE adds that the range may vary between local laboratories. Your printed report will show the reference range your own laboratory uses, and that range is the one to read your number against.
A result above the range suggests more active androgen than expected, and it is one of the findings a GP weighs when considering polycystic ovary syndrome. It is not a verdict by itself. NICE CKS lists it alongside total testosterone, SHBG and other hormone checks, and a clinician reads them together with your symptoms, your periods and sometimes an ultrasound scan.
Contraception changes the result
NICE CKS notes that a reliable assessment of excess androgens is not possible while taking hormonal contraception, because it changes SHBG. Tell your GP what you take before the blood test. Do not stop contraception on your own; your GP will advise whether a change is needed and what to use instead.
Why can the index be misleading?
The index can mislead because it depends on two moving parts and is only an estimate. Anything that shifts SHBG, such as hormonal contraception, thyroid changes, weight change or insulin resistance, moves the result even when testosterone itself is stable, and the ratio becomes less reliable when SHBG is very low.
Timing matters too. Hormones vary through the menstrual cycle and through the day, so a GP may ask for a test at a set time or repeat an unexpected result before acting on it. Illness, recent weight loss and some medicines can also skew a single reading.
There is also the question of what the number can tell you about your hair. DermNet says the majority of women with female pattern hair loss do not have underlying hormonal abnormalities, and that only a few are found to have excess androgens. So a normal result does not mean thinning is imagined, and a raised one does not prove it is the whole explanation.
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How does the FAI relate to hair?
The link to hair runs through androgen sensitivity in the hair follicle. DermNet explains that in pattern hair loss the hormone dihydrotestosterone shortens the growth (anagen) phase of scalp hair, while in other areas, such as the chin and upper lip, androgens encourage thicker, darker hair. Excess androgen can therefore thin the scalp and thicken body hair at once.
That pairing is why the NHS lists both excessive hair growth and hair loss among the main symptoms of polyendocrine metabolic ovarian syndrome, the new NHS name for polycystic ovary syndrome. It is also why a GP who sees thinning on top alongside new facial hair or acne is more likely to check hormones.
For a fuller look at this condition and hair, see our guide to PCOS hair loss and how to manage it, and for the testosterone side of the picture, our guide to testosterone and hair loss.
Is your hair problem loss, shedding or breakage?
Hair loss, hair shedding and hair breakage are separate problems, and only the first is closely tied to androgens. Hair loss here means pattern thinning from follicles shrinking over years; shedding means extra hairs falling from the root a few months after a trigger; breakage means strands snapping part-way along, leaving short ends.
| Problem | Typical sign | Link to androgen results |
|---|---|---|
| Pattern hair loss | Widening parting or thinning crown, gradual | Can be linked, but DermNet says most women have normal levels |
| Shedding (telogen effluvium) | Hairs with a white bulb, 2 to 4 months after illness, stress or childbirth | Usually unrelated; DermNet says it settles over 6 to 9 months |
| Breakage | Short, snapped ends and frizz, no bulb | Unrelated; comes from heat, colour and rough handling |
The NHS says it is normal to lose between 50 and 100 hairs a day. Counting hairs for a few days, and checking whether shed hairs have a bulb or a broken end, gives your GP useful detail alongside any blood result.
What can you do for your hair while you wait for results?
While you wait for results, the most useful thing you can do for your hair is reduce the daily losses you control: breakage from rough washing, tight styles and heat. A gentle routine will not change a hormone level, but it helps hair look fuller and keeps more length on your head while the cause is worked out.
- Wash with a mild, sulfate-free shampoo and focus it on the scalp.
- Condition every wash to cut tangles, the main cause of snapped strands.
- Use a wide-tooth comb on wet hair and avoid tight ponytails.
- Keep a note of when shedding started and any events two to four months earlier.
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When should you talk to a GP about your result?
You should talk to a GP about your result if it sits outside your laboratory's range, or if you have thinning hair with irregular periods, new facial or body hair, acne or weight change, because the NHS says these can be signs of PMOS and a GP can arrange further tests. See a GP sooner if hair changes come on quickly.
Take your printed results, a list of anything you take, including contraception and supplements, and your notes on when symptoms started. If your question is about products or a routine rather than a medical one, you can talk to Gail Waterman, our co-founder, as an AI avatar on the Watermans homepage. The index depends on your total result, and our guide to total testosterone explains what that number measures.
What are the frequently asked questions about the free androgen index?
What does a high free androgen index mean?
It suggests more active testosterone than expected. NICE CKS lists it among the checks for polycystic ovary syndrome, but a GP reads it with your symptoms and other results.
What is a normal FAI for a woman?
NICE CKS says the normal range is usually defined as lower than 5, but it varies between laboratories. Use the range printed on your report.
How is the FAI worked out?
NICE CKS gives it as 100 multiplied by total testosterone divided by SHBG, using results from the same blood sample.
Does the pill affect the result?
Yes. NICE CKS says hormonal contraception changes SHBG, so the result is not reliable while you take it. Do not stop it without advice.
Can a normal result still go with thinning hair?
Yes. DermNet says most women with female pattern hair loss have normal hormone levels, so thinning is worth discussing whatever the number.
Is the free androgen index used for men?
It is mainly used for women. Men have much higher testosterone, and labs usually use other measures for them.
What else can raise the index?
Anything that lowers SHBG, such as insulin resistance or weight gain, can raise the ratio even if testosterone is unchanged.
Can shampoo change my hormone levels?
No. A shampoo cannot change a blood result. A gentle routine helps hair look fuller and reduces breakage while your GP looks at the cause.
What is the bottom line on the free androgen index?
The bottom line on the free androgen index is that it is a useful estimate of active testosterone, calculated as 100 times testosterone over SHBG, with NICE CKS putting normal usually below 5. Read it with your laboratory's own range, remember that contraception and SHBG changes skew it, and let your GP put it in context with your symptoms.
Whatever the number says, your daily routine still shapes how your hair looks. A gentle hair growth shampoo used with conditioner is a simple step that keeps thinning hair looking fuller while you get answers.
Sources & references
- NICE Clinical Knowledge Summaries, Polycystic ovary syndrome: investigations, checked 18 September 2026.
- NHS, Polyendocrine metabolic ovarian syndrome (PMOS), checked 18 September 2026.
- DermNet, Female pattern hair loss, checked 18 September 2026.
- DermNet, Male pattern hair loss, checked 18 September 2026.
- DermNet, Telogen effluvium (hair shedding), checked 18 September 2026.
- NHS, Hair loss, checked 18 September 2026.
Watermans products are 100% cosmetic and do not treat medical hair loss. Individual results may vary.
