Helvy
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Hormones
In the UK, the standard clinical (NHS) reference range for Free Testosterone is 0.17–0.66 nmol/L, with 0.30–0.55 nmol/L considered the performance-optimised range. A result within these ranges suggests typical status; only a qualified clinician can interpret an individual reading.
Free testosterone is the fraction of total testosterone that circulates unbound to proteins — neither attached to sex hormone-binding globulin (SHBG, which binds roughly 65%) nor to albumin (which loosely binds roughly 33%). Only 1–3% of total testosterone flows freely in the blood, yet this is the only form that can cross cell membranes and activate androgen receptors in muscle, bone, brain, and reproductive tissue. Free testosterone is typically calculated from total testosterone, SHBG, and albumin using the Vermeulen equation — a validated method recommended by the Endocrine Society when direct equilibrium dialysis is unavailable. Because SHBG levels shift with age, body composition, thyroid function, and liver health, two people with identical total testosterone can have very different free testosterone — and very different symptoms.
Optimal range · UK
0.30–0.55 nmol/L
Performance-optimised band · clinical (NHS) range 0.17–0.66 nmol/L
Reference ranges for Free T, not a personal result. Any individual reading should be interpreted by a qualified clinician.
Optimal ranges
| Range | Value |
|---|---|
| Clinical (NHS) reference range | 0.17–0.66 nmol/L |
| Performance-optimised range | 0.30–0.55 nmol/L |
The clinical range defines what is considered medically “normal” — broad enough to cover 95% of the population. The performance range reflects where research and clinical experience suggest most people feel and function at their best. A result in either range suggests typical status and is not a diagnosis; any individual reading should be interpreted by a qualified clinician.
Why it matters
Free testosterone is often the missing piece when someone has symptoms of low testosterone — fatigue, poor recovery, low libido, brain fog — yet their total testosterone comes back 'normal'. This happens because SHBG rises with age, low calorie intake, hyperthyroidism, and certain medications (notably anticonvulsants and exogenous oestrogen), trapping more testosterone in its bound form and reducing the bioavailable fraction. Research published in the Journal of Clinical Endocrinology & Metabolism found that low free testosterone is associated with hypogonadal signs and symptoms in men whose total testosterone is within the normal reference range. Conversely, obesity and insulin resistance lower SHBG, which can inflate free testosterone while total testosterone falls — a pattern common in men with metabolic syndrome. For anyone focused on performance, body composition, or long-term vitality, free testosterone provides a far more accurate picture of androgen status than total testosterone alone. The European Academy of Andrology recommends calculating free testosterone whenever SHBG-altering conditions are present.
Symptoms
Low / Deficiency
High / Excess
Dietary sources
Supplementation
Free testosterone is not directly supplemented — it is improved by optimising total testosterone production and managing SHBG levels. Resistance training (compound lifts: squats, deadlifts, bench press) is the single most effective intervention for raising free testosterone naturally. Sleep is critical: even one night of 5-hour sleep reduces testosterone by 10–15% in healthy young men. Among supplements with evidence: zinc (30 mg/day if deficient), magnesium glycinate (200–400 mg daily — may reduce SHBG binding affinity), vitamin D3 (1,000–4,000 IU daily), and ashwagandha (600 mg KSM-66 — shown to raise total testosterone ~15% in stressed men). Boron (6–10 mg daily) has preliminary evidence for reducing SHBG and increasing free testosterone, though trials are small. Maintaining body fat between 12–20% is important — very low body fat raises SHBG (lowering free T), while obesity raises aromatase activity (converting testosterone to oestrogen). Avoid chronic caloric restriction, which reliably raises SHBG. Retest at 90 days alongside SHBG and total testosterone to track the ratio.
Testing
Free T is measured from a blood sample. With Helvy, that means a finger-prick kit taken at home and posted to a UKAS-accredited UK laboratory, with results in around 5 days, reviewed by a qualified clinician. Your result is reported against both the clinical range (0.17–0.66 nmol/L) and the performance-optimal range (0.30–0.55 nmol/L), so you can see not just whether you are “normal” but whether you are optimal. If you make a change, retest after 8-12 weeks to confirm it worked.
Research
Low Free Testosterone Is Associated with Hypogonadal Signs and Symptoms in Men with Normal Total Testosterone
Giagulli VA, Castellana M, Murro I, et al.
Journal of Clinical Endocrinology & Metabolism (2016)
DOI: 10.1210/jc.2015-4106Related biomarkers
Related guides
This content is for educational purposes only and does not constitute medical advice. Your data suggests areas for optimisation, but any concerns should be discussed with a qualified healthcare professional. If your results flag values outside safe ranges, we recommend consulting your GP.
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