Optimal Ranges
Clinical (NHS) Range
0.17–0.66 nmol/L
nmol/L
Performance-Optimised Range
0.30–0.55 nmol/L
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.
Why It Matters
Why Free T matters for performance
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
Signs your levels may be off
Low / Deficiency
- Fatigue and low energy despite adequate sleep
- Reduced muscle mass and difficulty building strength
- Increased body fat, particularly around the midsection
- Low libido and erectile difficulties
- Brain fog, poor concentration, and low motivation
- Depressed mood and irritability
- Slow recovery from exercise
High / Excess
- Acne and oily skin
- Hair loss (androgenic alopecia pattern)
- Aggression and mood instability
- In women: hirsutism, irregular cycles, and PCOS-related symptoms
Dietary Sources
Foods that support Free T levels
Supplementation
Evidence-based 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.
Research
Key study
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
Test your Free T levels
Free T is included in the Helvy 50+ biomarker panel. Get your results in 5 days with a personalised protocol.
Order Your TestThis 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.