Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
What is free testosterone? the unbound fraction
Free testosterone is the small fraction of your total testosterone that circulates unbound to any carrier protein — usually about 1–3% of the total — and it is the part that can enter cells and act on the androgen receptor. The rest is tied up: roughly 40–60% bound tightly to sex hormone-binding globulin (SHBG) and most of the remainder bound loosely to albumin, which is why two men with the same total testosterone can have very different free levels. This guide explains how the three fractions differ, walks through worked examples of the Vermeulen calculation, covers typical ranges, and answers the questions people ask most.
Key takeaways
- Free T = testosterone bound to nothing. Only ~1–3% of total is free.
- Bound T = SHBG-bound (tight, unavailable) + albumin-bound (loose).
- Bioavailable T = free + albumin-bound (the part tissues can reach).
- High SHBG hides testosterone; low SHBG frees it. Total can look "normal" while free is low.
- Estimate yours with the free testosterone index calculator using total T, SHBG and albumin.
What "free" actually means
When testosterone leaves the testes and adrenal glands it does not float through the bloodstream alone. Most of it — about 98% — immediately binds to two plasma proteins: SHBG and albumin (StatPearls, Androgen Replacement). SHBG binds testosterone with high affinity, almost like a lock, so SHBG-bound hormone is held off-limits to tissue. Albumin binds it with low affinity, more like a loose handshake that releases easily as blood passes through a capillary bed. The tiny slice left over, bound to nothing at all, is free testosterone.
Free testosterone matters because only unbound (and easily released albumin-bound) hormone can diffuse into a cell and activate the androgen receptor (StatPearls, Physiology, Testosterone). Total testosterone counts every molecule, available or not. That is why the question people really mean when they ask "what is free testosterone" is: how much of my testosterone is actually working?
Free vs bioavailable vs total
Three terms get used loosely. Total testosterone is everything in the sample. Free testosterone is the unbound fraction only. Bioavailable testosterone is free plus albumin-bound — the pool tissues can practically access — and it usually runs around 35–50% of total. The single number that shifts all three apart is SHBG.
Total vs free vs bioavailable testosterone
| Fraction | What it includes | Approx % of total | Available to tissue? |
|---|---|---|---|
| Total | Free + SHBG-bound + albumin-bound | 100% | Mixed |
| SHBG-bound | Tightly bound to SHBG | ~40–60% | No |
| Albumin-bound | Loosely bound to albumin | ~35–50% | Yes (releases readily) |
| Free | Unbound to any protein | ~1–3% | Yes |
| Bioavailable | Free + albumin-bound | ~35–50% | Yes |
The percentages are typical adult-male figures and shift with SHBG, albumin and age — treat them as a map, not a measurement. A lab measures free testosterone directly by equilibrium dialysis, or estimates it from total T, SHBG and albumin using a validated formula (Vermeulen 1999). Both are accepted; direct analog immunoassays for free T are less reliable and discouraged.
Why the free fraction matters
The Endocrine Society advises measuring free testosterone when total T sits near the lower limit of normal, or whenever a condition alters SHBG (Bhasin 2018). The reason is clinical: SHBG is not constant. It rises with age, hyperthyroidism, liver disease, estrogen and very low calorie states; it falls with obesity, type 2 diabetes, hypothyroidism and high insulin. When SHBG rises, more testosterone gets locked away and free T drops even if total looks healthy. When SHBG falls, free T can be adequate despite a modest total.
This is why a single total testosterone number can mislead. A lean older man with high SHBG and a man with metabolic syndrome and low SHBG can report the same total testosterone and feel completely different, because their free fractions differ. The free number — or the calculated free androgen index — resolves the mismatch.
Typical ranges
Total testosterone in healthy non-obese men aged 19–39 was harmonized at 264–916 ng/dL across four major cohorts (Travison 2017). Free testosterone has no single universal range — it depends on the assay — but calculated free T in eugonadal men commonly falls near 5–9 ng/dL (roughly 50–90 ng/L, or about 174–310 pmol/L), typically reported as ~1–3% of total. Always read free T against your own lab's reference interval and units, because methods and cut-offs vary.
| Marker | Typical adult-male reference | Common units |
|---|---|---|
| Total testosterone | 264–916 | ng/dL |
| SHBG | ~10–57 | nmol/L |
| Albumin | ~3.5–5.0 | g/dL |
| Calculated free T | ~5–9 (lab-dependent) | ng/dL |
How free testosterone is calculated
The Vermeulen method models the binding equilibrium between testosterone, SHBG and albumin using their known association constants, then solves for the unbound concentration (Vermeulen 1999). You do not do this by hand — it is a quadratic — but the inputs are simple: total testosterone, SHBG, and albumin (often assumed at 4.3 g/dL if not measured). The free testosterone index calculator runs the arithmetic for you. The worked examples below show the direction the answer moves, not the exact dialysis figure.
Total T = 600 ng/dL. If free T is ~2% of total, that is 600 × 0.02 = 12 ng/dL free. At a more conservative 1.5%, it is 600 × 0.015 = 9 ng/dL.
Two men both have total T = 500 ng/dL. Man A has SHBG 25 nmol/L, Man B has SHBG 55 nmol/L. More of B's testosterone is locked to SHBG, so B's free fraction is smaller — the same total, a lower free result.
Total T = 350 ng/dL (low-normal) but SHBG is only 12 nmol/L. With little SHBG to bind it, the free fraction climbs — calculated free T can land in range despite an unremarkable total.
A free T of 90 ng/L equals 9 ng/dL (divide ng/L by 10). In SI units, 9 ng/dL × 0.0347 ≈ 0.31 nmol/L, i.e. ~310 pmol/L. Same value, three notations.
If calculated free T = 9 ng/dL and total T = 600 ng/dL, percent free = 9 / 600 = 0.015 = 1.5% free. This sits squarely in the typical 1–3% band.
If free T is 1.5% and albumin-bound is ~40% of total, bioavailable = free + albumin-bound ≈ 1.5% + 40% = ~41.5% of total. On a 600 ng/dL total that is roughly 249 ng/dL tissue-available.
Formulas default albumin to 4.3 g/dL when it is not measured. If real albumin is 3.0 g/dL (low), less testosterone is albumin-bound, so the calculated free fraction shifts — one reason to measure albumin in liver or kidney patients rather than assume it.
So, what is free testosterone?
Free testosterone is the roughly 1–3% of total testosterone that is unbound to any protein and therefore able to enter cells and activate the androgen receptor. The rest is locked to SHBG or loosely held by albumin. Because SHBG shifts with age, thyroid, liver health, and body composition, free testosterone gives a clearer read on how much hormone your tissues can actually use than total testosterone alone. Use the free testosterone index calculator to estimate your free fraction from your total T, SHBG, and albumin values.
FAQs
What is free testosterone, in one sentence?
What is the difference between total and free testosterone?
Is free or total testosterone more important?
What is a normal free testosterone level?
Why is my total testosterone normal but free testosterone low?
Sources
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018. PubMed 29562364.
- Vermeulen A, et al. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999. PubMed 10523012.
- Travison TG, et al. Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies. J Clin Endocrinol Metab. 2017. PubMed 28324103.
- StatPearls. Androgen Replacement. NCBI Bookshelf. NBK534853.
- StatPearls. Physiology, Testosterone. NCBI Bookshelf. NBK526128.
This guide is for general educational purposes only and does not constitute medical advice. Reference ranges and free-testosterone methods vary by laboratory — always interpret your results against your own lab's reference interval and with your prescriber.