What Is Free Testosterone?
Free testosterone is the fraction of total testosterone that circulates unbound to any protein in the blood - typically 1–3% of the total. Unlike testosterone bound to SHBG (which is biologically inactive) or albumin (partially active), free testosterone can directly enter cells and activate androgen receptors, making it the most potent measure of androgenic activity.
Free testosterone is particularly important when SHBG is abnormal. A man with high SHBG may have normal total testosterone but low free testosterone - and symptoms of hypogonadism. Conversely, a woman with low SHBG may have normal total testosterone but elevated free testosterone - explaining her androgenic symptoms.
Free testosterone can be measured directly by equilibrium dialysis (the gold standard but expensive and slow) or calculated from total testosterone and SHBG using the Vermeulen equation. The free androgen index (FAI) is a simpler calculation more commonly used in women.
Why Is Free Testosterone Tested?
- Borderline total testosterone - free testosterone clarifies the clinical picture when total testosterone is equivocal
- Suspected androgen deficiency with high SHBG - ageing men, hyperthyroidism, liver disease, or OCP use raise SHBG
- Women with suspected androgen excess - free testosterone or FAI is more sensitive than total testosterone for PCOS
- Monitoring TRT - free testosterone helps optimise dosing
Normal Ranges
| Group | Normal Range |
|---|---|
| Adult males 20–39 | 0.198–0.619 nmol/L |
| Adult males 40–59 | 0.170–0.502 nmol/L |
| Adult males 60+ | 0.123–0.396 nmol/L |
| Adult females | 0.003–0.030 nmol/L |
Calculated free testosterone using the Vermeulen equation requires total testosterone, SHBG, and albumin. Reference ranges vary by method; always compare with the laboratory's own range.
Check Your Free Testosterone Levels at Home
The Hormone 7 Blood Test includes Free Testosterone testing along with other key biomarkers. Results in 2 working days with a free at-home phlebotomist visit.
View Hormone 7 Blood Test →What Do High Free Testosterone Levels Mean?
- Low SHBG - obesity, insulin resistance, hypothyroidism lower SHBG and raise free testosterone
- PCOS (women) - elevated free testosterone is often the earliest biochemical sign
- Exogenous testosterone, TRT or anabolic steroids
- Androgen-secreting tumours, adrenal or gonadal
What Do Low Free Testosterone Levels Mean?
- High SHBG - hyperthyroidism, liver disease, ageing, OCP use bind more testosterone
- Primary or secondary hypogonadism, same causes as low total testosterone
- Ageing, free testosterone declines faster than total because SHBG rises with age
- Chronic illness - diabetes, obesity, renal disease
How to Optimise Free Testosterone
- All the strategies for total testosterone apply - exercise, sleep, stress management, body composition
- Address SHBG abnormalities - treat thyroid dysfunction, reduce alcohol, manage insulin resistance
- Review medications — OCP and anticonvulsants raise SHBG; discuss alternatives if symptomatic
- Weight management — losing excess fat reduces insulin and raises SHBG toward normal, while also boosting total testosterone production
When Should You Get Tested?
- Total testosterone is borderline (8–12 nmol/L in men)
- Symptoms of androgen deficiency despite normal total testosterone
- Known conditions that alter SHBG (thyroid disease, liver disease, obesity)
- Women with androgenic symptoms and normal total testosterone
Which Lola Health Tests Include Free Testosterone?
Free testosterone is calculated from total testosterone and SHBG, both included in our Hormone 7 panel. Available as a specific add-on with any blood test.
Check Your Free Testosterone Levels
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This biomarker is included in our Testosterone Test and Male Hormones Clarity 14 — results in 2-3 working days with GP-reviewed insights.
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