Free vs Total Testosterone, SHBG and FAI: Understanding Your Results
What the four numbers on a testosterone test actually mean, why free testosterone often matters more than total, and how SHBG can make a normal result misleading.
Check your testosteroneA testosterone test measures four things: total testosterone, free testosterone, SHBG and the Free Androgen Index (FAI). Total is all the testosterone in your blood. Free is the small active fraction your body can actually use. SHBG is the protein that binds testosterone and decides how much is free. FAI is a calculation that estimates your active level. The key point is that free testosterone often matters more than total, because a normal total can sit alongside a low free level when SHBG is high, which a single number would miss. That is why a good testosterone test measures all four.
- Total testosterone is all of it, bound and free combined
- Free testosterone is the active fraction you actually use, often the more telling number
- SHBG is the binding protein that controls how much testosterone is free
- FAI (Free Androgen Index) is a calculation estimating your active androgen level
- A normal total with high SHBG can still mean low free testosterone and real symptoms
- SHBG rises with age, so free testosterone can fall faster than total
- A good test measures all four, not just total testosterone
The four numbers on a testosterone test
Most people think a testosterone test gives you one number. A good one gives you four, and understanding them is the difference between a result that actually explains how you feel and a number that leaves you guessing. Here is what each measures, then why the relationship between them matters more than any single figure.
Total testosterone
Total testosterone is the headline number: all the testosterone circulating in your blood, both the portion bound to proteins and the small free portion. It is the figure most often quoted, and a typical adult male range is roughly 8 to 30 nmol/L. But total testosterone has a blind spot. Most of it, often around 98%, is bound to proteins and therefore not available to your cells. So total tells you how much testosterone you have, not how much you can actually use. That is why a total reading on its own can look reassuring while you still feel every symptom of a low level.
Free testosterone
Free testosterone is the small fraction, usually a couple of per cent, that is not bound to any protein and is free to enter your cells and do its work. This is the testosterone your body actually uses, so it often tracks how you feel more closely than total does. Indicative free testosterone for adult men is roughly 255 to 725 pmol/L, varying by assay. When people describe a "normal total but low free" result, this is the number that has dropped, and it is frequently the one that explains fatigue, low libido or low mood despite a total that looks fine.
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View the Testosterone TestSHBG (sex hormone binding globulin)
SHBG is the protein that binds most of your testosterone and controls how much stays free. Think of it as a sponge: the more SHBG you have, the more testosterone it soaks up and holds unavailable, lowering your free level even when total is normal. Low SHBG does the opposite, freeing up more. SHBG is the hidden variable that makes total testosterone misleading, and it is why measuring it is essential rather than optional.
Several things move SHBG:
- Age, SHBG tends to rise as you get older, which is why free testosterone can fall faster than total with age
- Thyroid, an overactive thyroid raises SHBG
- Liver function, since SHBG is made in the liver
- Insulin resistance and obesity, which tend to lower SHBG
- Oestrogen and some medications
Free Androgen Index (FAI)
The Free Androgen Index is not a separate blood measurement but a calculation, total testosterone divided by SHBG, then multiplied out, that estimates how much active androgen you have. It is a quick way to account for SHBG without a direct free testosterone assay, and it is particularly useful when assessing androgen excess in women, for example in polycystic ovary syndrome (PCOS). On your report it sits alongside the measured values as another lens on the same question: how much usable testosterone do you have?
Free vs total testosterone: why the difference matters
This is the heart of it. Total and free testosterone answer different questions, and the gap between them is where most confusion lives.
| Total testosterone | Free testosterone | |
|---|---|---|
| What it is | All testosterone, bound and free | The small unbound, active fraction |
| What it tells you | How much you have | How much you can actually use |
| Affected by SHBG? | No | Yes, strongly |
| Tracks symptoms? | Sometimes | Often more closely |
The classic pattern: a man has a total testosterone of 15 nmol/L, comfortably "normal", but a high SHBG has pushed his free testosterone to the bottom of the range, and he has every symptom of a low level. Measure only total, and you would wrongly conclude everything is fine. Measure free and SHBG too, and the real picture appears. This single scenario is the best argument for never relying on a one-marker testosterone test.
See the full picture in one panel
View the Male Hormone ProfileHow to read them together
Read your results as a picture, not a list. Start with total for the overall level, then look at SHBG to see whether your testosterone is likely bound up or freed, then check free testosterone and FAI for the active amount. A doctor interpreting your report will weigh all four against your symptoms, age and history. If your total is normal but you feel low, the answer is often in the SHBG and free values. For how these change over the years, see normal testosterone levels by age, and for the practical steps of getting tested, how to check your testosterone levels.
Whichever test you choose, make sure it measures more than total. The focused Testosterone Test covers total, free, SHBG and FAI, while the Male Hormone Profile adds oestradiol, LH and FSH for the complete hormonal context. Browse the range under men's health.
Bottom line: total testosterone tells you how much you have, free testosterone tells you how much you can use, and SHBG explains the gap. A result is best interpreted across all four by a doctor, alongside how you feel.
Frequently asked questions
Total testosterone is all the testosterone in your blood, both bound and free. Free testosterone is the small active fraction not bound to proteins, which your body can actually use. Total tells you how much you have, free tells you how much is usable.
Both matter, but free testosterone often tracks how you feel more closely, because it is the active fraction. A normal total can sit alongside a low free level when SHBG is high, which is why a good test measures both plus SHBG.
SHBG (sex hormone binding globulin) is the protein that binds most of your testosterone and controls how much stays free and active. High SHBG lowers your free testosterone even when total is normal, and it tends to rise with age.
Indicative free testosterone for adult men is roughly 255 to 725 pmol/L, though it varies by laboratory and assay. It is the active fraction, so it is often more informative than total testosterone on its own.
FAI is a calculation from total testosterone and SHBG that estimates how much active androgen you have. It is a quick way to account for SHBG, and it is especially useful when assessing androgen excess in women, such as in PCOS.
Not always, some only report total. That is why it is worth choosing a test that measures free testosterone and SHBG as well. The Testosterone Test covers total, free, SHBG and FAI.
- Healthy Male (Andrology Australia). Understanding testosterone results. healthymale.org.au
- Royal Australian College of General Practitioners (RACGP). Testosterone, SHBG and free androgen index. racgp.org.au
- Endocrine Society of Australia (Hormones Australia). Testosterone and binding proteins. hormones-australia.org.au
- Healthdirect Australia. Testosterone blood tests. healthdirect.gov.au
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