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The free androgen index (FAI) estimates how much of your testosterone is free rather than bound, by comparing your total testosterone against SHBG. It's a rough proxy, and a widely misunderstood one: UK labs report it for women and report a calculated free testosterone for men instead. This is what your number means, and what to ask for.
What the free androgen index actually is (without the textbook)
Picture the printout. Three lines, one of them circled, and a GP who's already reaching for the door. "Testosterone's fine. Within range. Probably stress, maybe your sleep." You go home no clearer than you arrived, still flat by 2pm, still wondering why the number that was supposed to explain everything explained nothing.
That's where most men meet this topic. Not in a textbook. On a results page they can't fully read.
So before you spend another afternoon Googling symptoms, there's one thing worth getting straight. Your GP measured how much testosterone is in your blood. What governs how you feel is how much of it your body can actually use. Those aren't the same number.
The free androgen index is one attempt at the second one. It's a calculation, not a separate hormone. It compares your total testosterone against your SHBG and gives a quick estimate of the proportion that's free. The formula is simple: total testosterone divided by SHBG, times 100.
Your total testosterone is how much is in the tank. The free fraction is closer to how much the engine can actually reach.
Most men never get the second figure in any form. And a full tank with a blocked fuel line still leaves you stranded.
Why a "normal" total testosterone can still leave you low
Most of the testosterone in your blood isn't available to you. It's bound.
In men, about 45% is bound tightly to SHBG, and most of the rest is loosely attached to a protein called albumin. Only around 1 to 3% is completely free (Lab Tests Online UK, Sex hormone binding globulin). That free slice, plus the loosely albumin-bound part your body can still prise off, is what actually reaches your tissues and does the work.
Testosterone bound tightly to SHBG is locked up. Present in the total, absent from the job.
So follow what happens when SHBG runs high. More of your total gets tied up. The free fraction shrinks. And your total testosterone can still read comfortably mid-range the whole time, because the total counts the locked-up testosterone and the free testosterone as if they were the same thing.
They aren't the same thing. One you have. The other you can use.
This is the gap between "your total is normal" and "you still feel off". It isn't necessarily a testosterone-production problem. Often it's a measurement problem: the number that was taken wasn't the number that governs how you feel. A man in this position didn't get a low-testosterone result. He got a half-finished one.
One honest limit before we go further. That gap only opens when SHBG is unusual. When your SHBG sits inside its reference range, a calculated free testosterone tells you nothing your total hasn't already told you (Jayasena et al., 2023, SfE and ACB joint position statement). The free-fraction figures earn their keep when SHBG is high or low, not as a routine upgrade on every test.
SHBG: the protein that decides how much testosterone you can use
SHBG is the thing doing the deciding here, so it's worth understanding on its own.
What SHBG does
SHBG stands for sex hormone binding globulin. It's a protein made mainly in your liver, and its job is to bind tightly to sex hormones, testosterone included, and carry them around your bloodstream in an inactive form (Lab Tests Online UK, Sex hormone binding globulin). It also binds DHT and oestradiol.
Think of it as a set of handcuffs for your testosterone. Cuffed testosterone is still in your blood, still counted in your total, but it can't act on anything. The more SHBG you're carrying, the more of your testosterone is cuffed and the less is free to do its work.
Two men with an identical total testosterone can feel worlds apart, purely because one has high SHBG holding most of his hostage and the other doesn't.
What pushes SHBG up (and traps your testosterone)
A handful of things nudge SHBG higher. Liver disease, an overactive thyroid, higher oestrogen levels (including from some hormone medicines), certain anticonvulsant medicines, and ageing itself all tend to raise it (Lab Tests Online UK, Sex hormone binding globulin).
Higher SHBG, lower free fraction, for exactly the same total. That's the trap this article keeps returning to.
It's also why a man in his late 40s can have a total that's held up fine over the years and still feel the drop. His total didn't move much. His SHBG crept up, and his usable testosterone quietly fell behind it.
What pulls SHBG down
The mirror image also happens. Carrying excess weight and having an underactive thyroid both tend to lower SHBG (Lab Tests Online UK, Sex hormone binding globulin).
Low SHBG can make a total testosterone look lower than your free fraction really is, because less of your testosterone is bound. It's the opposite distortion to the trap. Both directions are the reason SHBG belongs on the panel: without it, you can't tell which way your total is being bent.
Total, free, bioavailable: three numbers, one hormone
You'll see three versions of "your testosterone" on a full panel. They describe the same hormone at different stages of being available to you.
What is free testosterone
Free testosterone is the small fraction, roughly 1 to 3% in men, that isn't bound to anything and can act directly on your tissues (Lab Tests Online UK, Sex hormone binding globulin). It's the biologically active portion. When people talk about the testosterone that actually reaches muscle, mood, libido and energy, this is the one they mean.
Bioavailable testosterone is a slightly wider figure: the free part plus the loosely albumin-bound part your body can still get at. Free plus reachable, in other words.
Calculated free testosterone vs the free androgen index
Both try to answer the same question. They don't do it equally well.
The free androgen index is the quick version, and quick is the whole of its appeal: total testosterone against SHBG, two numbers, one ratio.
A calculated free testosterone uses total testosterone, SHBG and albumin together, because albumin is where a big share of your reachable testosterone is loosely held (Vermeulen et al., 1999, free testosterone estimation). Leave albumin out and the calculation is incomplete.
That's the honest reason a good testosterone panel measures albumin. Not to pad the list. Without it, the calculated free testosterone doesn't add up.
Why UK labs don't report the free androgen index for men
FAI is not the number to chase. It's a proxy for one, and not a very good one in men.
When researchers compared it against calculated free testosterone in men, the two agreed poorly: an r squared of 0.21 to 0.46, meaning FAI explained less than half the variation in the figure it's standing in for. It also tended to overestimate at low SHBG concentrations, which is exactly the situation where you most want an accurate answer. The paper's own recommendation was to use calculated free testosterone when a total testosterone result is ambiguous, rather than relying on FAI (Ho et al., 2006, Calculated free testosterone in men).
UK labs go the same way in practice. North Bristol NHS Trust, whose reference ranges are quoted further down this page, spells out what it reports when testosterone and SHBG are ordered together: "calculated Free Androgen Index (Females) and calculated Free Testosterone (Males)" (North Bristol NHS Trust, SHBG). FAI is the figure they report for women.
I found this out the slow way. I ordered the panel the internet recommends, and my own NHS lab measured the components, calculated the index, and then printed a line underneath saying the test isn't considered valid in men because testosterone varies through the day. A different objection to the correlation one. It pointed the same way.
So the useful version of this isn't "get your FAI measured". If your total testosterone is ambiguous and your SHBG is unusual, ask for a calculated free testosterone. If you already have an FAI on a report, read it as a rough signal, not as your usable number.
What counts as a low free androgen index: the UK numbers
This is the part search engines quote and worried men screenshot. So here it is plainly, with the caveats attached rather than buried.
The SHBG reference range
UK labs report an adult male SHBG band that's genuinely wide. North Bristol NHS Trust, for example, uses 13 to 90 nmol/L for men (North Bristol NHS Trust, SHBG). Other UK labs draw the line elsewhere, some as tight as 14 to 71.
That spread isn't sloppiness. Each lab sets its range from its own tested population and its own assay, so where you fall matters relative to the range on your report, not to a universal number.
The FAI reference range, and why it's shakier than it looks
A band of roughly 30 to 150 is widely quoted for adult men, with values below 30 said to point towards low androgen activity (Health Screening Clinic, Free androgen index guide). Because FAI is a ratio, it carries no units.
Worth knowing where that band comes from. It's published by private testing services rather than set by a UK guideline, and it's quoted far more often than it's sourced. No UK guideline defines a male FAI action range, which follows from the section above: labs that don't report the figure for men have no reason to band it for men either.
For reference, a UK adult male total testosterone range is around 6 to 27 nmol/L (North Bristol NHS Trust, Testosterone).
Why the number is a signal, not a verdict
A single result is a snapshot. It's shaped by when you tested, whether you'd been recently ill, and which assay ran it.
UK guidance frames these calculations as a refinement, not a label. The British Society for Sexual Medicine advises checking SHBG and free testosterone specifically when total testosterone sits in the borderline 8 to 12 nmol/L range (Hackett et al., 2023, BSSM testosterone deficiency guidelines). That's where the free-fraction figures earn their keep, and it's a narrower claim than the one this article used to make.
"Total testosterone is the number most men are given, and on its own it can be reassuring for the wrong reasons. If SHBG is high, a normal total can still leave very little testosterone free to do its work. The free androgen index, and a properly calculated free testosterone, are what let you see that. It doesn't diagnose anything by itself, but it asks a better question than the total ever could."
Dr Ewa Lindo, GMC-registered GP, Andro Prime medical reviewer
This isn't the "normal range is a myth" argument (and why both matter)
There are two separate problems with the way testosterone gets judged, and it's easy to blur them into one.
The first is that the reference range itself is a statistical band, built to catch illness rather than to describe wellness. That's a real problem, and it has its own home. We make that case in full in The Myth of the Normal Range, so we won't re-run it here.
This article is about the other problem. Not that the ruler is wrong. That they measured the wrong thing with it.
When your GP tests total testosterone and stops there, the reference-range debate never even gets a chance to apply, because your usable figure was never on the page. One article says the ruler is off. This one says they measured the wrong quantity. Complementary, not competing.
And if your usable testosterone does turn out to be genuinely low and confirmed, what to actually do about it is a separate question again. We cover the lifestyle side of that in how to increase testosterone naturally. This article's job stops at helping you see the number clearly.
Why the time of day you tested changes the whole picture
Testosterone runs on a daily rhythm. It's highest in the early morning and drifts down as the day goes on. UK guidance is to take the sample in the morning, before 11am, and to confirm a low reading on a second morning test rather than acting on one (Hackett et al., 2023, BSSM testosterone deficiency guidelines). NHS labs ask for morning collection for the same reason (North Bristol NHS Trust, Testosterone).
It's also the objection my lab raised above: a figure built on a number that moves through the day inherits that movement.
So think about the man tested at 4pm. His number was drawn at his daily low, then never had its free fraction calculated properly. Two ways the same test can quietly mislead him, stacked on top of each other.
An at-home finger-prick test puts the timing back in your hands. You pick the morning slot, you're not squeezed into whatever appointment the surgery had free. That's a factual advantage, not a cure for anything.
What a single result does and doesn't tell you
One test is a starting point, not a full stop.
A low free fraction doesn't diagnose a condition. It's a signal that says "look closer", not a label that says "here's what you've got".
It also doesn't, on its own, mean you need testosterone replacement therapy. TRT is a prescribed clinical therapy, decided by a doctor on the whole picture, and it isn't something Andro Prime offers. It's a medical decision made with a clinician, not a product you buy off the back of one blood test.
What a good result does give you is a clearer question to take forward. Symptoms, plus a normal total, plus an SHBG that explains why the total is misleading, is a far stronger thing to walk into an appointment with than a vague sense that something's off.
When it's your testosterone, and when it's your GP
That line doesn't move.
This test helps you find out whether it's your testosterone. It does not treat anything. That's the honest promise, and being straight about it is the point.
If your total testosterone comes back below 12 nmol/L, the next step is your GP, not a product. There's no kit and no supplement being sold into that result. A total in that territory is a conversation for a doctor, and UK guidance regards the borderline 8 to 12 nmol/L band as a reason for further assessment, not reassurance (Hackett et al., 2023, BSSM testosterone deficiency guidelines). We'd rather point you there than keep you.
Where this test earns its place is the other case: a normal total, an SHBG that's high or low, a calculated free testosterone that comes in under it, and symptoms you recognise. That's the picture worth taking to a GP with the actual numbers, instead of being turned away on the total alone.
And some situations skip us entirely. If you're under 18, if you have a known liver or thyroid condition, if you take a medicine that affects SHBG, or if your symptoms genuinely worry you, that's your GP, not a wellness test.
How Andro Prime measures your usable testosterone (Kit 1)
The Testosterone Health Check measures Total testosterone, SHBG, Albumin and a calculated Free testosterone: the full set needed to work out how much of your testosterone is usable, not just the single total most tests report. Your Free Androgen Index is reported alongside them, as a reported figure rather than one we band or make a recommendation on. It's a finger-prick testosterone blood test you take at home, processed by a UKAS ISO 15189-accredited lab, with results in 2 to 5 working days.
Your results come back with thresholds and plain-English context built on Ewa-approved recommendation logic. We don't diagnose, and we don't interpret your results one by one. We measure your levels, calculate the usable fraction, and give you a clear next step, including the GP-referral one when a result calls for it.
Your next move
If you only read this section: your total testosterone counts the locked-up testosterone and the usable testosterone as one number. Measuring SHBG alongside it tells you which way your total is being bent. The free androgen index is a rough proxy for the usable fraction and UK labs report it for women, not men; the figure to ask for is a calculated free testosterone, and it only adds information when your SHBG is high or low. A single reading is a signal, not a verdict, and anything below 12 nmol/L total is a GP conversation, not a wellness one.
So here's the question worth sitting with.
If your GP measured how much testosterone you have but never checked what your SHBG was doing to it, what would actually change once you saw both? You can keep guessing whether "normal" was normal for you. Or you can find out which way your total is being bent, and walk into the next conversation with the figures they skipped.
Find out how much of your testosterone you can actually use. The Testosterone Health Check measures Total testosterone, SHBG, Albumin and a calculated Free testosterone, and reports your Free Androgen Index alongside them: the full picture, not just the single number most tests report. Finger-prick at home, UKAS ISO 15189-accredited lab, results in 2 to 5 working days.
References
- Lab Tests Online UK (Association for Laboratory Medicine, with the Royal College of Pathologists and the Institute of Biomedical Science). Sex hormone binding globulin (SHBG). https://labtestsonline.org.uk/tests/sex-hormone-binding-globulin-shbg
- North Bristol NHS Trust, Severn Pathology. Testosterone test information (adult male reference range 6 to 27 nmol/L; morning sampling). https://www.nbt.nhs.uk/severn-pathology/requesting/test-information/testosterone
- North Bristol NHS Trust, Severn Pathology. SHBG test information (adult male reference range 13 to 90 nmol/L; calculated Free Androgen Index reported for females and calculated Free Testosterone for males). https://www.nbt.nhs.uk/severn-pathology/requesting/test-information/shbg
- Ho, C. K. M., Stoddart, M., Walton, M., Anderson, R. A., Beckett, G. J. (2006). Calculated free testosterone in men: comparison of four equations and with free androgen index. Annals of Clinical Biochemistry 43(Pt 5): 389 to 397. https://pubmed.ncbi.nlm.nih.gov/17036414/
- Jayasena, C. N., de Silva, N. L., O'Reilly, M. W., et al. (2023). Standardising the biochemical confirmation of adult male hypogonadism: a joint position statement by the Society for Endocrinology and Association of Clinical Biochemistry and Laboratory Medicine. Annals of Clinical Biochemistry 60(4): 223 to 227. https://journals.sagepub.com/doi/10.1177/00045632231179022
- Health Screening Clinic. How to use the free androgen index: a calculation and interpretation guide (the widely quoted adult male FAI band of approximately 30 to 150; a private-provider figure, not a UK guideline). https://www.healthscreeningclinic.co.uk/blog/how-to-use-free-androgen-index-calculation-interpretation-guide
- Hackett, G. et al. (2023). The British Society for Sexual Medicine Guidelines on Male Adult Testosterone Deficiency, with Statements for Practice. World Journal of Men's Health 41(3): 508 to 537. https://doi.org/10.5534/wjmh.221027
- Vermeulen, A., Verdonck, L., Kaufman, J. M. (1999). A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology & Metabolism 84(10): 3666 to 3672. https://pubmed.ncbi.nlm.nih.gov/10523012/
Frequently asked questions
What is the free androgen index (FAI)?
The free androgen index is a calculation that estimates how much of your testosterone is free rather than bound. It compares your total testosterone against your SHBG, the protein that binds testosterone and holds it in an inactive form. The formula is total testosterone divided by SHBG, multiplied by 100. It's a rough index of the free fraction, not a separate hormone and not a diagnosis. UK labs generally report it for women and report a calculated free testosterone for men instead (North Bristol NHS Trust, SHBG), because FAI correlates poorly with calculated free testosterone (Ho et al., 2006).
What is a normal free androgen index for men in the UK?
There isn't an established one. A band of roughly 30 to 150 is widely quoted by private testing services, with values below 30 said to point towards low androgen activity, but no UK guideline sets a male FAI action range, and UK NHS labs typically don't report FAI for men at all (North Bristol NHS Trust, SHBG). FAI is a ratio, so it carries no units. If your report shows one, read it against the range printed on that report, and rely on the calculated free testosterone as the more reliable figure.
What is SHBG and what does it do?
SHBG stands for sex hormone binding globulin. It's a protein made mainly in your liver that binds tightly to testosterone (and to DHT and oestradiol) and carries it around in an inactive, unavailable form (Lab Tests Online UK, Sex hormone binding globulin). In men, about 45% of testosterone is bound tightly to SHBG. Only around 1 to 3% is completely free. The more SHBG you have, the more of your testosterone is tied up and out of reach.
What causes high SHBG?
SHBG tends to be pushed up by liver disease, an overactive thyroid (hyperthyroidism), higher oestrogen levels, some anticonvulsant medicines, and by ageing (Lab Tests Online UK, Sex hormone binding globulin). Higher SHBG means more of your testosterone is bound and less is free, so your calculated free testosterone can be low even when your total testosterone reads mid-range. That's the exact pattern this guide exists to explain.
What is free testosterone?
Free testosterone is the small fraction of your testosterone, roughly 1 to 3% in men, that isn't bound to any protein and can act on your tissues (Lab Tests Online UK, Sex hormone binding globulin). It's the biologically active portion. Bioavailable testosterone is a slightly wider figure: the free part plus the loosely albumin-bound part your body can still get at. A calculated free testosterone uses total testosterone, SHBG and albumin together (Vermeulen et al., 1999), and it's the derived figure UK labs report for men.
Can your total testosterone be normal but your free testosterone low?
Yes, and it's common when SHBG is high. Most of your testosterone is bound to SHBG and albumin, and only the free fraction can do any work. If your SHBG is high, a larger share of your total is locked up, so your free testosterone can sit low while your total number looks reassuringly mid-range. A total-only test can't show you that. The caveat runs the other way too: when SHBG is inside its reference range, a calculated free testosterone adds nothing beyond the total (Jayasena et al., 2023).
What's the difference between total and free testosterone?
Total testosterone is everything in your blood: the bound and the free added together. Free testosterone is only the small unbound part that can act on your tissues. Think of total as how much is in the tank and free as how much the engine can actually reach. Two men with the same total can have very different free levels, depending on their SHBG.
How do you test the free androgen index?
You can't get either figure from a total testosterone test alone. FAI needs total testosterone and SHBG measured together, and a calculated free testosterone also needs albumin (Vermeulen et al., 1999). Andro Prime's Testosterone Health Check measures Total testosterone, SHBG, Albumin and a calculated Free testosterone from a finger-prick sample, and reports your Free Androgen Index alongside them. It helps you find out whether your usable testosterone is low. It doesn't treat anything, and if your total comes back below 12 nmol/L the next step is your GP.