Free testosterone
HormonesAlso known as: free T, fT, calculated free testosterone, bioavailable testosterone (calculated), unbound testosterone
A calculated value: free testosterone is estimated from total testosterone, SHBG and albumin (Vermeulen equation) to gauge the small, biologically active fraction of testosterone not bound to carrier proteins.
The unbound, biologically active fraction of total testosterone that can interact with cellular receptors. This measurement provides insight into androgenic activity affecting metabolism, body composition, and various physiological functions. In this panel it is a calculated value: rather than being measured directly, it is derived from total testosterone, SHBG and albumin using the Vermeulen equation (the laboratory reference method is equilibrium dialysis). Because the calculation corrects for SHBG, free testosterone is the guideline-preferred measure of androgen status when SHBG is abnormal or total testosterone sits near the lower limit of normal, and it is more sensitive than total testosterone for detecting male hypogonadism and female androgen excess such as PCOS.
Reference range
Female
Source: Ahead Health benchmark
Male
Source: Ahead Health benchmark
Reference ranges may vary between labs and assays. Always interpret results with your healthcare provider.
Why this matters
Free testosterone levels provide insight into hormone activity beyond total testosterone. Low free testosterone can impair strength, bone health, and sexual function even if total testosterone is normal. Lifestyle factors such as weight management, exercise, and sleep help maintain healthy free testosterone levels.
How this connects to other biomarkers
- Free Testosterone is the biologically active fraction — it can be low even when Testosteron, total is normal, if Sex hormone-binding globulin (SHBG) is high (overactive thyroid, oral estrogen, liver disease).
- Low free Testosterone with symptoms warrants distinguishing primary (high Luteinizing hormone (Lutropin) / FSH, Follitropin) from secondary (low gonadotropins) low testosterone.
- In women, elevated free Testosterone with low SHBG and high Free Androgen Index (FAI) points to PCOS (polycystic ovary syndrome) or other androgen excess.
How often should I test Free testosterone?
For most adults an annual check — alongside total testosterone and SHBG, since it is calculated from them — is enough. If you are being assessed or treated for low testosterone, retest about 6–12 weeks after starting or changing therapy and then every 6–12 months once stable, always on a fasting early-morning sample.
At baseline / for screening: Every 12 months for most adults, on an early-morning sample (before 11am).
When monitoring an intervention or change: Every 6–12 weeks after starting or adjusting testosterone therapy, until levels are stable.
Note: Testosterone follows a strong circadian rhythm and falls after eating, so sample fasting before 11am. Because free testosterone here is calculated from total testosterone, SHBG and albumin, anything that shifts SHBG — thyroid disease, obesity, insulin resistance, liver disease, oestrogens — will move the calculated result; confirm borderline values with a repeat early-morning sample or direct equilibrium-dialysis measurement.
Sources
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