Free Testosterone
The unbound, bioactive testosterone fraction, typically 1-4% of total T. Correlates with symptoms better than total testosterone alone.
TL;DR. Normal adult male: ~9-30 ng/dL (Vermeulen-calculated). Declines ~1-2% per year after 30. Best calculated from total T + SHBG + albumin using the Vermeulen 1999 quadratic. Direct measurement by equilibrium dialysis is gold standard but expensive; direct immunoassay is unreliable and discouraged.
Definition
Free testosterone is the fraction of circulating testosterone that is not bound to SHBG or albumin. It can diffuse into tissues and activate androgen receptors, making it the closest-to-bioactive clinical measure. Typically 1-4% of total T in men; the rest is bound (40-60% to SHBG with high affinity, 35-50% to albumin loosely).
Reference range (adult males, Vermeulen-calculated)
| Age band | Free T (ng/dL) |
|---|---|
| 20-29 | 15-30 |
| 30-39 | 14-28 |
| 40-49 | 12-25 |
| 50-59 | 10-22 |
| 60-69 | 8-20 |
| ≥70 | 7-18 |
Free T percentage typically 1.5-3.0% of total T.
What high free T means
- TRT or AAS use, expected.
- Low SHBG states without exogenous input, metabolic syndrome, obesity, hyperinsulinemia.
- Testosterone-secreting tumor, rare, typically with elevated total T and DHEA-S.
- Assay artifact, possible but uncommon with Vermeulen-calculated values.
What low free T means
- Primary or secondary hypogonadism.
- High SHBG (hyperthyroidism, aging, liver disease, anti-convulsants, anorexia), total T normal but free T low.
- Opioid-induced hypogonadism.
- Age-related decline, gradual.
- Severe illness, caloric deficit, sleep deprivation, transient.
When to test
- Whenever you measure total testosterone, pair with SHBG + albumin for Vermeulen calculation.
- When total T and symptoms disagree.
- Baseline and follow-up on TRT.
- Any workup of suspected hypogonadism.
Related tests
FAQ
- What is the normal range for free testosterone?
- Approximately 9-30 ng/dL for adult men (Vermeulen-calculated). Age-indexed: 20s-30s typically 15-30; 40s-50s 10-25; 60s-70s 8-20; ≥70 7-18. Free T percentage typically 1.5-3.0% of total T. Ranges are laboratory-dependent.
- Why is free T more informative than total T?
- Because symptoms track free T closely: it is the fraction actually available to diffuse into tissue and bind androgen receptors. Two men with identical total T can have dramatically different symptomatic androgen status if their SHBG differs, free T collapses that variance into a single clinically meaningful number.
- How is free testosterone measured?
- Three methods: (1) equilibrium dialysis, gold standard, expensive; (2) direct analogue immunoassay, inaccurate at low values, Endocrine Society advises against; (3) calculated from total T + SHBG + albumin using the Vermeulen 1999 quadratic, the clinical default, correlates r ≥ 0.95 with equilibrium dialysis.
- What does low free testosterone mean?
- Low free T with symptoms (low libido, fatigue, erectile dysfunction, mood changes, reduced strength) is the clinical definition of hypogonadism. Causes range from primary/secondary hypogonadism to age-related decline, high SHBG from thyroid excess, obesity-related HPG suppression, or chronic illness. Asymptomatic low free T is typically not treated.
- What does high free testosterone mean?
- Supraphysiologic free T can indicate TRT or anabolic use (expected), low SHBG states without exogenous input (metabolic syndrome), or rarely testosterone-secreting tumors. Symptoms of very high free T may include acne, elevated hematocrit, aggression, sleep disruption, and accelerated hair loss in genetically predisposed men.