HORMONES & PERFORMANCE
Free Testosterone vs Total: Which Matters More?
QUICK ANSWER
Free testosterone is the 2 to 3% of testosterone not bound to SHBG or albumin, and it is the fraction your cells can actually use. BSSM guidance treats calculated free testosterone below 0.225 nmol/L as low and above 0.30 nmol/L as healthy, so total testosterone can look normal while free testosterone is deficient.
Two men get the same blood result: total testosterone 16 nmol/L. One feels strong. The other has the full set of low-testosterone symptoms, and his GP can't explain why. The difference usually comes down to a number most basic tests never report: free testosterone.
This guide explains what free testosterone is, the levels that count as normal in UK units, how SHBG decides how much of your testosterone actually works, and when measuring it changes the answer. It draws on the 2023 BSSM guidelines and peer-reviewed research throughout.
By Helvy Medical Team · 12 min read
1. Bound vs free: what's actually in your blood
Total testosterone counts every molecule of the hormone in your blood. But most of it is carried, not working. Around 60–70% is bound tightly to sex hormone-binding globulin (SHBG), a protein made by the liver. Another 30–40% is loosely attached to albumin. Only 2–3% travels unbound.
That unbound fraction is free testosterone. It is the only part that can enter cells, bind to androgen receptors, and do the things testosterone is known for: building muscle, maintaining bone density, supporting libido, mood, and drive. SHBG-bound testosterone can't do any of that. It is cargo in transit.
This is why two men with identical total testosterone can feel completely different. If one has twice the SHBG of the other, he has roughly half the active hormone, despite a matching headline number. Our guide to testosterone levels by age covers the total ranges in detail; this guide covers the fraction that actually reaches your cells.
2. SHBG: the dial that sets your free testosterone
SHBG behaves like a dial. Turn it up and more testosterone gets locked away; turn it down and more runs free. A typical adult male range is roughly 18–54 nmol/L, though labs vary. What moves the dial matters more than the number itself; our dedicated SHBG guide covers high and low results in depth.
WHAT RAISES SHBG (LOWERS FREE T)
Ageing, an overactive thyroid, liver disease, some anticonvulsant medications, oestrogen therapy, and sustained low body weight or aggressive calorie restriction. Endurance athletes with very low body fat often run high SHBG.
WHAT LOWERS SHBG (RAISES FREE T)
Obesity, insulin resistance and type 2 diabetes, an underactive thyroid, and anabolic steroid use. Low SHBG isn't a win: it is one of the earliest blood markers of metabolic dysfunction, and it often appears years before HbA1c drifts out of range.
The practical point is simple. A testosterone result without SHBG is half a result. The British Society for Sexual Medicine recommends checking free testosterone whenever total testosterone sits in the grey zone (8–12 nmol/L) or whenever SHBG is likely to be abnormal. In practice, that covers a lot of the men who feel symptomatic with a “normal” result.
3. Normal free testosterone levels (UK units)
UK labs report free testosterone in nmol/L, and the numbers are small. Don't let that throw you. A free testosterone of 0.3 nmol/L is healthy; the same figure for total testosterone would be alarming. The thresholds below come from the BSSM 2023 guidance.
| FREE TESTOSTERONE | INTERPRETATION |
|---|---|
| >0.30 nmol/L | Healthy. Androgen deficiency very unlikely to explain symptoms |
| 0.225–0.30 nmol/L | Borderline. Worth a repeat morning test and a look at SHBG, sleep, and body composition |
| <0.225 nmol/L | Low by BSSM criteria. With symptoms, this supports a diagnosis of testosterone deficiency even when total T is “normal” |
One caveat. Free testosterone has no single universal reference range, because the result depends on the method used to derive it. That is why the BSSM anchors decisions to the 0.225 nmol/L cut-off from calculated free testosterone, and why the next section matters.
4. Calculated vs measured free testosterone
Directly measuring free testosterone is hard. The gold-standard method, equilibrium dialysis, is slow and expensive, so almost no UK lab uses it routinely. The older direct immunoassays are considered unreliable, and the BSSM guidelines advise against relying on them.
Instead, labs calculate it. The standard approach is the Vermeulen equation, validated in a 1999 study in the Journal of Clinical Endocrinology & Metabolism, which derives free testosterone from three measured values: total testosterone, SHBG, and albumin. Calculated free testosterone tracks equilibrium dialysis closely and is the form the BSSM thresholds are built on.
The practical consequence: you cannot get a meaningful free testosterone figure from a test that only measures total testosterone. The panel has to include SHBG. If you have old results that include both, free testosterone can be calculated retrospectively; if your test only reported total T, the information simply isn't there.
5. Four result patterns and what they mean
Once you have total testosterone, SHBG, and calculated free testosterone side by side, most results fall into one of four patterns.
| PATTERN | WHAT IT SUGGESTS |
|---|---|
| Normal total, normal free | Testosterone is unlikely to be the problem. Look at thyroid, iron, vitamin D, sleep, and cortisol instead |
| Normal total, low free | High SHBG is locking testosterone away. The classic missed case: symptoms are real, total T looks fine. Investigate what is driving SHBG up |
| Low total, normal free | Low SHBG, usually from excess weight or insulin resistance, is propping up the free fraction. The hormone result is a metabolic warning, not just a hormonal one |
| Low total, low free | Genuine testosterone deficiency. LH and FSH then show whether the cause sits in the testes or the pituitary, which changes the treatment conversation entirely |
Pattern two is the one this guide exists for. If you have the symptoms described in our guide to low testosterone in men but you've been told your level is normal, ask one question: was that total or free?
6. How to test free testosterone properly
The same timing rules apply as for any testosterone test. Morning sample, ideally before 10am, fasted. Testosterone peaks between 7 and 10am and can be 30% lower by late afternoon, which is enough to turn a borderline result into a falsely low one. A single low reading should always be confirmed with a repeat test at least four weeks later.
For the result to be interpretable, the panel needs, at minimum:
- Total testosterone— the headline number
- SHBG — without it, free testosterone cannot be calculated
- Free testosterone (calculated) — the biologically active fraction
- LH and FSH— to locate the cause if levels are low
Context markers earn their place too. Oestradiol shows how much testosterone is being converted by body fat, prolactin flags pituitary causes, and cortisol and DHEA-S capture the stress side of the equation. Our Complete Male Hormones panel (£119) measures all nine: total and free testosterone, SHBG, LH, FSH, prolactin, oestradiol, DHEA-S, and cortisol, from one morning finger-prick sample.
7. Can you raise free testosterone?
Yes, but not by chasing the number directly. Free testosterone rises when you either produce more testosterone or stop locking it away, and both routes run through the same unglamorous territory.
Body composition comes first. Losing visceral fat lowers the aromatase activity that drains testosterone into oestradiol, and improving insulin sensitivity normalises SHBG from the other direction. Sleep is second: a study published in JAMA found one week of five-hour nights cut testosterone by 10–15% in healthy young men. Resistance training, less alcohol, and managing chronic stress make up the rest. We cover the evidence for each in our guide to raising testosterone naturally.
Supplements help only when they correct a documented deficiency. Vitamin D, zinc, and magnesium all support testosterone production when your blood shows you are short of them. Taken on top of normal levels, they do very little. Test first. Then supplement what the numbers say you need, not what the marketing says you might. Our zinc blood test guide covers why a normal-looking result can still hide a shortfall.
8. Free androgen index: the version that matters for women
In women, the question flips. Clinicians usually want to know whether the active testosterone fraction is too high, because excess free androgens drive the acne, excess hair growth, and irregular cycles seen in polycystic ovary syndrome (PCOS).
The standard tool is the free androgen index (FAI): total testosterone divided by SHBG, multiplied by 100. An FAI above roughly 5 suggests androgen excess in women, though lab cut-offs vary. Because the calculation depends on SHBG, the same rule applies as for men: a testosterone result without SHBG can't answer the question being asked.
Our Hormone Balance panel (£99) measures testosterone, SHBG, and the free androgen index alongside FSH and LH, the combination used in PCOS investigation and perimenopause assessment.
9. Frequently asked questions
What is a normal free testosterone level for a man?
Above 0.30 nmol/L is generally healthy. The BSSM considers calculated free testosterone below 0.225 nmol/L low, and values between the two are borderline and worth repeating. UK labs report in nmol/L; if you are reading American sources, their pg/mL figures are not directly comparable without conversion.
Can free testosterone be low when total testosterone is normal?
Yes, and it is common. High SHBG binds more of your testosterone, leaving less of it free to act. Ageing, thyroid overactivity, liver conditions, and very low body fat all push SHBG up. A man with total testosterone of 16 nmol/L and SHBG of 70 nmol/L can have genuinely deficient free testosterone despite a normal-looking headline result.
Is calculated free testosterone accurate?
Calculated free testosterone using the Vermeulen equation correlates closely with equilibrium dialysis, the laboratory gold standard, and it is the method BSSM thresholds are based on. It needs accurate total testosterone and SHBG inputs, which is why both markers appear together on any worthwhile hormone panel.
Does the NHS test free testosterone?
Not usually as a first step. A standard NHS workup measures total testosterone, and SHBG or calculated free testosterone is typically added only if the first result is borderline and your GP pursues it. The NHS pathway can take several visits to reach the full picture a single comprehensive panel provides on day one.
What is a good free androgen index for a woman?
Most UK labs consider an FAI below roughly 5 normal for women, with higher values suggesting androgen excess and supporting a PCOS workup when symptoms fit. Cut-offs vary by lab, so the reference range on your report takes precedence.
Should I test free testosterone or just total?
If you are testing because of symptoms, test both, plus SHBG. Total testosterone alone misses the high-SHBG pattern entirely, and that pattern is precisely the one that leaves men symptomatic with “normal” results. The cost difference is small; the difference in what the result can tell you is not.
Measure what actually matters
The Complete Male Hormones panel (£119) measures total and free testosterone, SHBG, LH, FSH, prolactin, oestradiol, DHEA-S and cortisol. Home finger-prick kit, results in 5 days, processed by UKAS-accredited UK laboratories.
Medical disclaimer: This content is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Reference ranges and thresholds cited in this guide are based on BSSM guidelines and published research, and may differ from the ranges used by your local laboratory. Do not make changes to medication, supplementation, or treatment plans based solely on information in this article; consult your GP or a qualified healthcare professional. All Helvy blood tests are processed by UKAS-accredited UK laboratories to ISO 15189.
Last updated: June 2026 · By Helvy Medical Team · Analysed at UKAS-accredited UK laboratories