Free vs. Total Testosterone: Which Number Actually Matters?
Both matter — but not equally, and not always at the same time. Here's how to read free vs. total testosterone and when each one drives the decision.
— TL;DR
Total testosterone is the easier number to measure; free testosterone is the biologically active one. In most men with normal SHBG, they move together. In men with abnormal SHBG (high or low), they can diverge dramatically — and treating the total number while ignoring free leads to bad decisions. Free testosterone drives symptoms; total is the initial screen.
— Key takeaways
- Total testosterone includes SHBG-bound, albumin-bound, and free fractions.
- Free testosterone is the 1-2% that's actually bioactive.
- Symptoms correlate more strongly with free T than with total T.
- Obesity lowers SHBG, which makes total T look deceptively okay.
- Aging raises SHBG, which makes total T look better than free T performs.
Disclosure. This page contains affiliate links. If you click through and make a purchase, we may earn a commission at no additional cost to you. Full disclosure
Why There Are Three Numbers
In your bloodstream, testosterone exists in three forms:
- SHBG-bound (~45-65% of total) — tightly bound to sex hormone-binding globulin, inactive
- Albumin-bound (~30-55%) — loosely bound; can dissociate and enter cells
- Free (~1-2%) — completely unbound, readily enters cells and binds androgen receptors
"Total testosterone" is all three added together. "Free testosterone" is just the last fraction. "Bioavailable testosterone" is free plus albumin-bound (the fraction your tissues can actually access).
Only the free and bioavailable fractions are biologically active. The SHBG-bound fraction is like money locked in a trust — technically yours, practically unavailable.
Why Total Testosterone Is the Default Screen Anyway
Total testosterone has two things going for it as a screen:
- Cheap and easy to measure — a routine chemiluminescence immunoassay does it reliably
- Well-standardized ranges across labs and decades of clinical data
If your total testosterone is clearly low (below 250) or clearly high (above 700), the free number usually tracks. The issue is the middle zone — total T in the 300-450 range — where SHBG variability makes total an unreliable proxy for free.
When Total and Free Diverge (The Useful Part)
High SHBG pushes total up, free down
Causes: aging, hyperthyroidism, liver disease, anorexia, oral estrogens.
A 65-year-old with total T of 410 ng/dL and SHBG of 72 nmol/L probably has free T around 6 ng/dL — low. His total looks borderline-normal; his free is clearly low. Treating him based on total alone would miss the diagnosis.
Low SHBG pulls total down, free up
Causes: obesity, insulin resistance, type 2 diabetes, hypothyroidism, nephrotic syndrome, steroid use.
A 35-year-old with total T of 345 ng/dL and SHBG of 14 nmol/L probably has free T around 9 ng/dL — within range. His total looks low; his free is actually adequate. Treating him on total alone could be overtreating.
This is why a thoughtful clinician orders total + free + SHBG together, not sequentially.
“Total testosterone is a screen. Free testosterone is the diagnostic. Men with normal totals and low free are the ones who most often get missed.”
How Free Testosterone Gets Measured
Three main methods, roughly in order of reliability:
Equilibrium dialysis (gold standard)
Physically separates free from bound testosterone, then measures the free fraction directly. Available at reference labs (Quest, LabCorp Esoteric, Endocrine Sciences). Expensive ($80-150 cash-pay); worth it in ambiguous cases.
Calculated free testosterone (most common)
Uses the Vermeulen equation: free T = f(total T, SHBG, albumin). Highly correlated with equilibrium dialysis in most men; breaks down a bit in men with very high or very low SHBG. This is what most commercial labs report.
Direct "free T" immunoassay (avoid)
Older technique that measures what it claims is free testosterone. Poor correlation with actual free T. Most guidelines recommend against this method. If your report says "free testosterone" without a SHBG value, check whether it was calculated or direct immunoassay — the latter is essentially useless.
How to Read Your Results
Four common scenarios:
1. Total low + free low + SHBG normal
Clear hypogonadism. Order LH/FSH next to determine primary vs. secondary.
2. Total normal + free low + SHBG high
Functional hypogonadism driven by high SHBG. Common in older men. Treatment is still reasonable if symptoms are present; the total number is a red herring.
3. Total low + free normal + SHBG low
Metabolic pattern — obesity, insulin resistance, often type 2 diabetes. The low SHBG is inflating free T relative to total. Treatment question is nuanced; lifestyle intervention should come first in many of these men.
4. Total normal + free normal + symptoms
Hormones probably aren't your problem. Look at sleep, thyroid, mood, alcohol, overtraining. Don't treat normal numbers.
Where Free T Is Especially Critical
Aging men
SHBG tends to rise with age. A total T of 450 at 30 is different from a total T of 450 at 70 — the older man probably has less free T despite the same total.
Obese men
SHBG falls with rising insulin resistance. A total T of 290 in a 280-pound man may correspond to a relatively preserved free T, and losing 30 pounds may be more useful than starting testosterone.
Men on thyroid medication changes
Hyperthyroidism elevates SHBG; treating it (or over-replacing on levothyroxine) can move both total and free T meaningfully.
Men starting TRT
Tracking free T on treatment matters — total T can stay "in range" while free T runs high if SHBG is suppressed.
A Concrete Example
Consider two patients, both with total T of 380 ng/dL:
Patient A, 32, BMI 24, nonsmoker, athletic
SHBG: 48 nmol/L
Calculated free T: 8.2 ng/dL (normal)
Symptoms: minimal; one episode of low libido after a stressful quarter
→ Probably eugonadal. Watch and recheck in 3 months.
Patient B, 42, BMI 31, prediabetes, snores
SHBG: 14 nmol/L
Calculated free T: 6.1 ng/dL (low-normal)
Symptoms: fatigue, libido loss, brain fog, weight gain
→ Probably symptomatic low T with low SHBG masking the total. Consider lifestyle + cautious TRT with full workup.
Same total. Different diagnosis. Different treatment.
Bottom Line
Total testosterone is a useful starting point but a poor endpoint. Free testosterone correlates more tightly with symptoms because it's the fraction that actually does biological work. Always get SHBG with a testosterone test — without it, you're reading half the story. In ambiguous cases, equilibrium-dialysis free T is worth the extra cost to resolve the picture.
When a clinician tells you "your testosterone is normal" and you still feel hypogonadal, ask specifically for free T and SHBG. It's the single most common way a real diagnosis gets missed.
Sources
- Vermeulen A et al. "A Critical Evaluation of Simple Methods for the Estimation of Free Testosterone in Serum." J Clin Endocrinol Metab, 1999.
- Rosner W et al. "Position Statement: Utility, Limitations, and Pitfalls in Measuring Testosterone." J Clin Endocrinol Metab, 2007.
- Bhasin S et al. "Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline." J Clin Endocrinol Metab, 2018.
- Antonio L et al. "Low Free Testosterone Is Associated With Hypogonadal Signs and Symptoms in Men With Normal Total Testosterone." J Clin Endocrinol Metab, 2016.
- Goldman AL et al. "A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications." Endocr Rev, 2017.
Frequently asked questions
Ready to get your testosterone checked?
At-home blood test, physician consultation, and treatment — starting at $99/month.
Get Started with PeterMD→Medical Disclaimer. This content is for informational purposes only and does not constitute medical advice. Always consult a licensed healthcare provider before starting any treatment. TRT requires a prescription from a licensed physician.
— Read next
SHBG and Low Testosterone: The Most Ignored Number in Your Panel
Sex hormone-binding globulin decides how much of your testosterone is actually usable. Here's why it matters, what changes it, and what to do with the number.
Read →What Blood Tests You Actually Need Before Starting TRT
A full TRT workup is more than one testosterone number. Here's the panel a legitimate clinician orders, why each test matters, and what the results mean.
Read →Estradiol Levels on TRT: What's Normal, What's Not, and What to Do
Men need estrogen too. Here's what estradiol should actually be on TRT, how to read your labs, and when — or if — to use an aromatase inhibitor.
Read →