Testosterone blood test UK
Main testing guide.
Vitalicore • UK men over 40
Free vs total testosterone test: what the difference means in plain English
Total testosterone is a starting point. Free or calculated free testosterone can help explain borderline cases, but the result still needs clinical context.
Comparison table
Use this as a decision guide, not as a diagnosis.
| Marker | What it means | Why it matters |
|---|---|---|
| Total testosterone | Overall testosterone in blood | Common first marker |
| SHBG | Binding protein that changes interpretation | Helps explain mismatch |
| Free testosterone | More available fraction | Useful in some borderline cases |
| Repeat morning test | Confirms pattern | One result can mislead |
Next pages
Borderline testosterone
Unclear result path.
What the two numbers actually measure
Almost all the testosterone in your blood is bound to proteins and cannot act on tissue. Only the small unbound fraction is biologically active, which is why a total figure can be misleading on its own.
| Fraction | Roughly how much | Biologically active? |
|---|---|---|
| Bound to SHBG (sex hormone binding globulin) | Around 45–65% | No — bound tightly |
| Bound to albumin | Around 33–54% | Loosely bound, so partly available |
| Free (unbound) | Around 1–3% | Yes |
| Total testosterone | All of the above added together | The number most tests report |
Why SHBG is the number that explains confusing results
SHBG is the reason two men with identical total testosterone can feel completely different, and it is why a normal total result sometimes sits alongside genuine symptoms.
| SHBG | Effect on free testosterone | Commonly raised or lowered by |
|---|---|---|
| High SHBG | Less free testosterone available, so a normal-looking total can be misleading | Ageing, hyperthyroidism, liver disease, some medications, being very lean |
| Low SHBG | More free testosterone available, so a low-looking total may be less significant | Obesity, insulin resistance, type 2 diabetes, hypothyroidism, steroid use |
This is why UK practice asks for SHBG alongside total testosterone when the total is borderline. Without it, a borderline number cannot be interpreted.
Calculated free testosterone beats a directly measured one
There are two ways a lab can give you a free testosterone figure, and they are not equivalent.
- Calculated — derived from total testosterone, SHBG and albumin. Widely available, well validated, and what most NHS and good private panels use.
- Direct immunoassay — measured directly. Common on consumer panels and known to correlate poorly with the reference method.
- Equilibrium dialysis with mass spectrometry — the reference standard, but a specialist test you will rarely be offered.
Practically: if a panel offers "free testosterone" without also reporting SHBG and albumin, it is probably a direct assay. Choose the panel that gives you total, SHBG and albumin instead. See the most accurate testosterone test.
Which one should you ask for?
Ask for total testosterone with SHBG, taken in the morning. That combination lets free testosterone be calculated, and it is what a GP will typically order anyway.
Free testosterone becomes the more important figure when the total is borderline, when you are significantly overweight (which lowers SHBG), or when symptoms and the total result disagree. It is not a better test in general — it is the number that resolves ambiguity in the total.
Then read what a borderline result means and what to ask your GP.
FAQ
What is the difference between free and total testosterone?
Total testosterone is everything in your blood, most of which is bound to SHBG or albumin and cannot act on tissue. Free testosterone is the roughly 1–3% that is unbound and biologically active.
Should you test free or total testosterone?
Ask for total testosterone with SHBG, taken in the morning. That lets free testosterone be calculated, and it is what UK practice uses. Free testosterone matters most when the total is borderline or when symptoms and the total disagree.
Why does SHBG matter?
It determines how much testosterone is actually available. High SHBG means a normal-looking total can still leave you short of free testosterone; low SHBG, common with obesity and insulin resistance, means a low-looking total may be less significant.
Is calculated free testosterone accurate?
It is well validated and is what most NHS and good private panels use. It is more reliable than the direct free testosterone immunoassays sold on many consumer panels, which correlate poorly with the reference method.
Can you have normal total testosterone and still have symptoms?
Yes, and high SHBG is one common explanation — it leaves less free testosterone available despite an adequate total. That is exactly the situation SHBG testing exists to identify.