Total vs free, simply
Testosterone in the bloodstream exists in three forms:
- SHBG-bound (~60-70%), tightly attached to sex hormone-binding globulin. Cannot enter cells. Biologically inactive.
- Albumin-bound (~28-38%), loosely attached to albumin. Can dissociate easily and enter cells. Bioavailable.
- Free (~1-3%), circulating unbound. Immediately able to enter cells and bind androgen receptors.
Total testosterone, the standard test, sums all three. Free testosterone measures only the third. Bioavailable testosterone (less commonly tested) measures free + albumin-bound, the truly usable fraction.
The ratio is not fixed. Two men with identical total T levels can have very different free T levels depending on their SHBG. This is why the same number means different things in different bodies.
Why free is what matters
Cells don't see total testosterone. They see what crosses the cell membrane and binds androgen receptors, and that's overwhelmingly free T (with some contribution from albumin-bound). The SHBG-bound fraction is essentially a reservoir; it isn't doing anything until it dissociates.
This is why men with seemingly normal total T can have low-T symptoms (fatigue, low libido, poor recovery, depressed mood). The total looks fine, the body is starved at the cellular level. The reverse is also true: a relatively low total with healthy SHBG can leave plenty of free T in circulation, and the man feels fine.
The Endocrine Society's 2018 clinical practice guideline explicitly recommends measuring free T (or calculating it from total T and SHBG) in cases where total T is borderline or where SHBG is suspected to be abnormal (Bhasin et al., J Clin Endocrinol Metab 2018).
The role of SHBG
SHBG is produced by the liver and acts as the carrier protein for sex hormones. Its level is regulated by:
- Insulin (high insulin lowers SHBG)
- Thyroid hormone (high thyroid raises SHBG)
- Liver health (cirrhosis raises SHBG)
- Estrogen (oral estrogen dramatically raises SHBG; transdermal does not)
- Aging (SHBG rises with age in men)
- Diet (very low calorie / very high fiber / very low protein raise SHBG)
- Body fat (high body fat lowers SHBG)
This means a man's SHBG tells you something about his metabolic and hepatic state, beyond just acting as a testosterone modulator.
The Vermeulen equation
Direct measurement of free T (analog tracer methods) is unreliable on most platforms. The gold standard is equilibrium dialysis or, more practically, calculation using the Vermeulen formula (Vermeulen et al., J Clin Endocrinol Metab 1999):
Inputs needed:
• Total testosterone (ng/dL or nmol/L)
• SHBG (nmol/L)
• Albumin (g/dL, usually ~4.3)
Most labs report calculated free T automatically when you order total T + SHBG + albumin together.
The Vermeulen calculation correlates closely with equilibrium dialysis and is what most quality clinics use. Direct ELISA-based free T measurements should be viewed with skepticism unless the lab specifies LC-MS/MS methodology.
The four common patterns
| Total T | SHBG | Free T | Common cause + likely treatment |
|---|---|---|---|
| Low | Normal | Low | Classic hypogonadism, TRT or enclomiphene candidate |
| Low | Low | Low | Insulin resistance / obesity, address metabolic first; TRT often unnecessary |
| Normal | High | Low | Age-related, hyperthyroid, or oral estrogen, calculated free T is the diagnostic key |
| High | Normal | Normal-high | Healthy young man, no intervention needed |
Pattern 3, normal total, high SHBG, low free, is the most commonly missed in routine medicine. The patient is told his total T is fine, but he's symptomatic because his free T is low. Always ask for SHBG and calculated free T when total T is in the lower-to-middle range.
The clinical pearl: Two men, both with total testosterone of 500 ng/dL. Man A has SHBG of 25 (calculated free T ~12.5 ng/dL = healthy). Man B has SHBG of 75 (calculated free T ~6 ng/dL = clearly low). Their treatments and prognoses are completely different.
Optimal free T values
Reference ranges for free T are wide, typically 4.0-28.0 ng/dL depending on the lab. Optimal, drawn from healthy-population data and symptom-correlation studies:
- Optimal calculated free T (men): 15-25 ng/dL (ideally upper half)
- Symptomatic threshold: usually below 9 ng/dL
- Optimal calculated free T (women): 0.3-1.0 ng/dL
Free T scales with total T but is more sensitive to SHBG fluctuations. Year-over-year tracking of free T is more useful than tracking total T alone.
What to do if free T is low
If your total T is normal but free T is low, the strategy depends on which pattern you're in:
- Low SHBG + low free T: reverse insulin resistance. Lose visceral fat, lift weights, address sleep apnea, consider GLP-1 if appropriate. SHBG often normalizes and free T rises without TRT.
- High SHBG + low free T: address upstream causes, check thyroid (treat if hyperthyroid), reduce alcohol, ensure adequate protein, address liver health. If SHBG remains stubbornly high and total T isn't compensating, low-dose TRT often resolves symptoms because therapeutic TRT raises both total and free.
- Both total and free low + normal SHBG: classic hypogonadism. Consider enclomiphene (if secondary and fertility matters) or TRT.
The pattern dictates the protocol. Treating "low T" as a single diagnosis is one of the biggest oversights in routine endocrinology.
Bottom line
Free testosterone is the metric that actually predicts how you feel. Total T is useful but incomplete. Always order total T, SHBG, and albumin together so calculated free T is available, and read all three numbers in the context of one another. The single biggest cause of "low T symptoms with normal labs" is high SHBG, and it's invisible if you only look at total.