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The Free Androgen Index compares your total testosterone to your SHBG. Enter both in nmol/L to see your FAI and where it sits against commonly quoted bands.
Most of the testosterone in your blood is not free to do anything. A carrier protein holds it. That protein is sex hormone binding globulin, usually written as SHBG, and it behaves like a sponge. What the sponge does not soak up is the part your cells can use.
The free androgen index estimates that leftover part without measuring it directly. It divides total testosterone by SHBG, then multiplies by 100. Both numbers go in as nmol/L. That is the sum UK laboratories print, and it is the one the calculator above runs.
So the index answers one question. For the amount of carrier protein you are carrying, how much of your testosterone is spare?
Bands differ between laboratories, so the range printed on your own report is the one that counts. These are the bands commonly quoted in the UK, and the ones this calculator reads your value against.
| Who | Commonly quoted band | What sits outside it |
|---|---|---|
| Premenopausal women | Up to about 5 | Above that is described as raised. Some labs draw the line nearer 4.5. |
| Adult men | About 24 to 104 | Under 24 reads low. Over 104 reads raised. |
| After the menopause | Lower than the premenopausal band | The index falls naturally with age, so compare like with like. |
Here is the same testosterone twice. A total testosterone of 1.6 nmol/L with an SHBG of 45 gives an index of 3.6. The same 1.6 with an SHBG of 25 gives 6.4. The testosterone never moved.
Timing matters too. Total testosterone peaks in the early morning and drifts down through the day, and the index follows it. An afternoon sample reads lower than a morning one from the same person, which is why comparing two results taken at different times tells you very little.
Because it is a ratio, and SHBG is the bottom of it. Drop SHBG and the index climbs, even with a total testosterone sitting mid-range. Push SHBG up and the index falls the same way.
The commonest reason SHBG runs low is insulin. When the body resists insulin and levels climb, the liver makes less SHBG. Wallace and colleagues set that relationship out in Clinical Endocrinology in 2013. So a raised index sitting on a low SHBG often points at metabolic ground rather than at the ovaries or the testes. Our SHBG blood test guide walks through it, and a high SHBG with normal testosterone covers the opposite case.
That cuts both ways. SHBG also falls as weight rises, so an index raised on a low SHBG can be reading body composition as much as hormones. Worth knowing before you read much into one figure.
Less than it looks. A 2025 systematic review in Human Reproduction Update pooled the studies on androgen tests used when polycystic ovary syndrome (PCOS) is being assessed. In that pooled data the free androgen index flagged about 78 of every 100 women who had the condition, and correctly cleared about 85 of every 100 who did not.
Read that as useful rather than decisive. It misses roughly one in five. The same review puts the index alongside total testosterone and free testosterone, not instead of them.
And no blood number diagnoses PCOS on its own. That takes symptoms, scans and bloods read together by a qualified clinician. Our PCOS blood test guide sets out what the full assessment looks like.
Not quite. Free testosterone is the unbound hormone itself. A lab can measure it directly, or work it out from testosterone, SHBG and albumin. The index is the simpler cousin. It leaves albumin out and reports a ratio instead of a concentration.
The two move together, so a low SHBG lifts both. They do not read the same, though, and they are not interchangeable on a report. If yours gives free testosterone in pmol/L, our free testosterone calculator is the one you want, and what free testosterone measures explains the difference in full.
One practical note before you type anything in. The sum needs both values in nmol/L. If your testosterone came back in ng/dL, which US labs and most US charts use, convert it first with our nmol/L and ng/dL converter. Dividing by 28.842 does the same job. SHBG is reported in nmol/L nearly everywhere, so it usually needs nothing doing.
The index needs two results from the same sample: total testosterone and SHBG. Both of our hormone tests carry them, and both report the calculated index so you do not have to work it out. Hormone Balance is the one built around FSH, LH, SHBG and testosterone. Complete Male Hormones adds free testosterone, prolactin and DHEA-S.
Not sure which one fits? Build your test asks a few questions and points you at one.
Book an appointment if any of these apply, whatever the index says:
An index is information. A diagnosis is a conversation.
How this is worked out
FAI is calculated as (total testosterone ÷ SHBG) × 100, with both values in nmol/L, the standard formula printed by UK laboratories. Reference bands vary between labs: an FAI up to about 5 is a range commonly quoted for premenopausal women, and roughly 24 to 104 for adult men. Always read the range on your own report, and note that FAI falls naturally after menopause. This is reference context only, not a diagnosis.
This tool is for general wellbeing information and is not a diagnosis or medical advice. It places the numbers you enter in context; it cannot tell you whether you have a condition. If a result concerns you, or you have symptoms, speak to your GP.
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