Is Free Testosterone the Same as Bioavailable Testosterone? How Free, Bioavailable and Total T Differ on a Lab Report
Three numbers can show up on a hormone panel, and they measure three different slices of the same hormone. Which slice you look at can decide whether a man is told his levels are fine.
Free testosterone is not the same as bioavailable testosterone. Free T is the small unbound share, about 1% to 4% of the total. Bioavailable T is free T plus the testosterone loosely held by albumin, which is everything not locked up by SHBG.
Is free testosterone the same as bioavailable testosterone? No. Free testosterone is only the unbound hormone, and a 2017 review in Endocrine Reviews by Goldman, Bhasin and colleagues puts it at 1% to 4% of circulating testosterone. Bioavailable testosterone is a bigger number: free testosterone plus the testosterone loosely attached to albumin, a common blood protein. Both leave out the testosterone gripped tightly by sex hormone-binding globulin (SHBG).
The difference matters because a lab report can show total, free or bioavailable testosterone, and each one answers a different question. A man can have a normal total testosterone and a low free testosterone at the same time. In the European Male Ageing Study, 277 of 3334 men fell into exactly that gap, and they reported more symptoms than men whose numbers were normal across the board.
1% to 4%
Share of Circulating Testosterone That Is Free
The unbound fraction, Goldman 2017, Endocrine Reviews
33% to 54%
Share Loosely Bound to Albumin
Free plus albumin-bound testosterone makes up bioavailable T, Goldman 2017
277
EMAS Men With Normal Total T but Low Free T
Out of 3334 men aged 40 to 79; they had more sexual and physical symptoms, Antonio 2016, JCEM
2% vs 0.8%
Yearly Decline in Free vs Total Testosterone
Cross-sectional age trend, Massachusetts Male Aging Study, Feldman 2002
20-25%
How Much Lower Testosterone Ran at 4 PM vs 8 AM
Men aged 30 to 40, Brambilla 2009, JCEM
What Is the Difference Between Free and Bioavailable Testosterone?
Free testosterone is the testosterone that is not attached to any protein, while bioavailable testosterone is all testosterone that is not bound to SHBG. The Goldman review defines bioavailable testosterone as the fraction of circulating testosterone not bound to SHBG, and says it largely represents the sum of free testosterone plus albumin-bound testosterone. Free testosterone is therefore one part of bioavailable testosterone, never the other way round.
The split comes down to how tightly each protein holds on. According to the Goldman review, circulating testosterone is bound tightly to SHBG and weakly to albumin and two minor proteins, orosomucoid and corticosteroid-binding globulin. Testosterone on albumin can let go easily as blood passes through tissues, which is why researchers count it as available. Testosterone on SHBG is held much more firmly.
Illustration: Where a Man's Testosterone Sits in the Blood
Total
All testosterone
Every molecule in the sample, bound and unbound. This is the standard first test.
SHBG-bound
Held tightly
Locked to sex hormone-binding globulin. Excluded from both free and bioavailable counts.
Albumin-bound
Held loosely
Weakly attached to albumin and released easily. Counted in bioavailable testosterone.
Free
Unbound
Not attached to any protein. The smallest slice, and part of bioavailable testosterone.
Free vs Bioavailable vs Total Testosterone: What Each Number Counts
| Lab Number | What It Includes | Share of Total | How It Is Usually Obtained |
|---|---|---|---|
| Total testosterone | SHBG-bound + albumin-bound + free | All of it | Direct measurement; the first-line test |
| Bioavailable testosterone | Albumin-bound + free (non-SHBG) | Free plus the 33% to 54% on albumin | Ammonium sulfate precipitation, or calculated from free T |
| Free testosterone | Unbound only | 1% to 4% | Equilibrium dialysis, or calculated from total T and SHBG |
Is Free Testosterone the Same as Total Testosterone?
Free testosterone is not the same as total testosterone; it is a small fraction of it. Total testosterone counts every testosterone molecule in the sample, bound and unbound. In 9054 men from four cohort studies, Travison and colleagues set the harmonized normal range for total testosterone in healthy nonobese men aged 19 to 39 at 264 to 916 ng/dL (JCEM, 2017). Free testosterone is reported in much smaller units, usually pmol/L or pg/mL.
For measured free testosterone, a 2026 study in the Journal of Clinical Endocrinology and Metabolism by Walravens and colleagues tested 1202 men with equilibrium dialysis and mass spectrometry. The reference range in healthy nonobese men aged 18 to 39 was 184 to 749 pmol/L. Free testosterone fell by 4.5 pmol/L per year of age, and a BMI of 30 and 35 lowered it by 14.4% and 22.2% compared with a BMI of 22.
Is Free Testosterone Important?
Free testosterone is important because it tracks symptoms better than total testosterone in some men. Antonio and colleagues analysed 3334 community-dwelling men aged 40 to 79 in the European Male Ageing Study and published the results in the Journal of Clinical Endocrinology and Metabolism in 2016. They used a total testosterone cutoff of 10.5 nmol/L and a calculated free testosterone cutoff of 220 pmol/L, then compared four groups.
How 3334 Men Split by Total and Free Testosterone (EMAS)
The two mismatched groups told opposite stories. The 277 men with normal total testosterone (mean 14.2 nmol/L) but low free testosterone (mean 194 pmol/L) were older, had higher SHBG and LH, and reported more sexual and physical symptoms, with lower hemoglobin and weaker bone ultrasound results. The 96 men with low total testosterone (mean 9.6 nmol/L) but normal free testosterone (mean 247 pmol/L) were younger and more obese, had lower SHBG, and lacked features of androgen deficiency.
The authors concluded that low calculated free testosterone, even with normal total testosterone, is associated with androgen deficiency symptoms, and that free testosterone should be assessed in men with suspected hypogonadal symptoms. Our guide to SHBG and free vs total testosterone explains why a high SHBG can make a normal total number misleading.
Why SHBG Decides Which Number Matters
SHBG is the swing factor between total and free testosterone. When SHBG runs high, as it often does with age, more testosterone is locked away and free testosterone can be low while total looks fine. When SHBG runs low, as it often does with obesity, total testosterone can look low while free testosterone is adequate. That is why the Endocrine Society guideline asks for a free testosterone result when SHBG is likely to be abnormal.
Why Does Free Testosterone Fall Faster Than Total Testosterone With Age?
Free testosterone falls faster with age because SHBG rises as men get older, which locks up a growing share of the hormone. The Massachusetts Male Aging Study followed 1,709 men aged 40 to 70 at baseline, and 1,156 were seen again 7 to 10 years later. Feldman and colleagues reported in JCEM in 2002 that total testosterone declined cross-sectionally by 0.8% per year of age, while free and albumin-bound testosterone declined by about 2% per year and SHBG rose by 1.6% per year.
Yearly Change by Age in the Massachusetts Male Aging Study (Cross-Sectional)
Within the same men followed over time, the declines were steeper: 1.6% per year for total testosterone and 2-3% per year for bioavailable testosterone. Men in apparent good health, without chronic illness, obesity or heavy drinking, had androgen levels 10-15% higher. The gap between total and free testosterone tends to widen as a man ages, so a stable total number can hide a falling free number.
Can Free Testosterone Be Measured?
Free testosterone can be measured directly, but the reliable method is slow and expensive, so most labs calculate it instead. Vermeulen and colleagues compared the main methods in JCEM in 1999. Equilibrium dialysis for free testosterone and ammonium sulfate precipitation for bioavailable testosterone were reliable but too time consuming for clinical routine. Free testosterone calculated from total testosterone and SHBG matched equilibrium dialysis almost exactly, except in pregnancy.
Methods for Free and Bioavailable Testosterone: How Reliable Is Each?
| Method | What It Reports | Verdict in the Research |
|---|---|---|
| Equilibrium dialysis | Free testosterone | Reliable reference method; too slow and costly for routine use (Vermeulen 1999) |
| Ammonium sulfate precipitation | Bioavailable (non-SHBG) testosterone | Reliable index; also too time consuming for routine use (Vermeulen 1999) |
| Calculated free T (total T + SHBG) | Free testosterone, estimated | Close to dialysis in most conditions; calculators lack standardization (Vermeulen 1999; Nyamaah 2024) |
| Direct analog immunoassay | Free testosterone | Inaccurate and should not be used (Goldman 2017) |
| Free androgen index | Total T divided by SHBG | Not a reliable index of bioavailable T in men (Vermeulen 1999) |
The direct analog assay is the one to watch for on a lab order. Vermeulen's team found its values were only a fraction of the true free testosterone, and the fraction shifted with SHBG levels. The Goldman review states plainly that direct analog assays for free testosterone are inaccurate and should not be used. Calculated values have their own limits: Goldman's group found the common linear formula gave results systematically lower than equilibrium dialysis.
Can Free Testosterone Fluctuate?
Testosterone fluctuates through the day, which is why timing matters for any testosterone test. Brambilla and colleagues drew blood from 66 men aged 30 to 80 at three morning and three afternoon visits (JCEM, 2009). In men aged 30 to 40, testosterone was 20-25% lower at 4 PM than at 8 AM, and the gap shrank to about 10% by age 70. SHBG varied much less across the day.
Afternoon testing can create false lows. In the Brambilla study, 17 men who had at least one reading below 300 ng/dL after noon had normal testosterone at all three morning visits. Since free testosterone is usually calculated from total testosterone and SHBG, a low afternoon total drags the calculated free number down with it. Our guide to testing testosterone accurately covers timing, fasting and repeat testing.
Can Free Testosterone Be Increased?
Free testosterone can rise with weight loss in men carrying excess weight, according to a 2013 meta-analysis by Corona and colleagues in the European Journal of Endocrinology. The analysis included 24 studies of low-calorie diets and bariatric surgery. Both approaches were associated with significant increases in SHBG-bound and unbound testosterone, and the rise was larger in men who lost more weight, in younger men and in men without diabetes.
The Walravens data point the same way from the other direction: free testosterone ran 14.4% lower at a BMI of 30 and 22.2% lower at a BMI of 35 than at a BMI of 22. Testosterone therapy also raises free testosterone, but it is a prescription treatment with its own risks and is only considered after a confirmed diagnosis. This article does not recommend any treatment.
Is Free Testosterone T3 or T4?
Free testosterone has nothing to do with T3 or T4. T3 and T4 are thyroid hormones, and free T3 and free T4 are their unbound forms on a thyroid panel. The shared word free simply means unbound in both cases, and the letter T in testosterone shorthand is a coincidence. Thyroid problems can still affect SHBG and testosterone, which our article on thyroid function and testosterone explains.
Which Testosterone Test Should a Man Ask For?
Most men should start with total testosterone and add free testosterone when the result is borderline or SHBG may be abnormal. The 2018 Endocrine Society guideline by Bhasin and colleagues recommends a fasting morning total testosterone with an accurate assay as the first test, repeated to confirm. In men whose total testosterone is near the lower limit of normal or who have a condition that alters SHBG, the guideline recommends free testosterone by equilibrium dialysis or an accurate formula.
Illustration: A Sensible Order for Testosterone Testing
Step 1
- ✓Book a fasting blood draw in the morning
- ✓Ask which assay the lab uses
- ✓Repeat the test if the first result is low
Step 2
- ✓Ask for SHBG if total testosterone is borderline
- ✓Ask for free testosterone by equilibrium dialysis or a calculated value
- ✓Avoid the direct analog free testosterone assay
Step 3
- ✓Compare free testosterone with the lab's own reference range
- ✓Consider age, body weight and thyroid status, which all shift SHBG
- ✓Match the results to symptoms before drawing conclusions
Step 4
- ✓Look for causes before any treatment discussion
- ✓Recheck after major weight change
- ✓Keep copies of every result for comparison
This article is for educational purposes only and is not medical advice. Nothing here diagnoses, treats, cures or prevents any disease, including hypogonadism. Testosterone therapy is a prescription treatment discussed only in terms of what published research reported. Speak with a licensed physician before ordering tests, starting or changing any medication.
Sources
- Goldman AL et al. A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications. Endocrine Reviews, 2017. https://doi.org/10.1210/er.2017-00025
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology and Metabolism, 1999. https://doi.org/10.1210/jcem.84.10.6079
- Bhasin S et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology and Metabolism, 2018. https://doi.org/10.1210/jc.2018-00229
- Antonio L et al. Low Free Testosterone Is Associated with Hypogonadal Signs and Symptoms in Men with Normal Total Testosterone. Journal of Clinical Endocrinology and Metabolism, 2016. https://doi.org/10.1210/jc.2015-4106
- Feldman HA et al. Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts male aging study. Journal of Clinical Endocrinology and Metabolism, 2002. https://doi.org/10.1210/jcem.87.2.8201
- Brambilla DJ et al. The effect of diurnal variation on clinical measurement of serum testosterone and other sex hormone levels in men. Journal of Clinical Endocrinology and Metabolism, 2009. https://doi.org/10.1210/jc.2008-1902
- Travison TG et al. Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe. Journal of Clinical Endocrinology and Metabolism, 2017. https://doi.org/10.1210/jc.2016-2935
- Walravens J et al. Age-Stratified Reference Ranges for Directly Measured Serum Free Testosterone in Community-Dwelling and Healthy Men. Journal of Clinical Endocrinology and Metabolism, 2026. https://doi.org/10.1210/clinem/dgaf507
- Corona G et al. Body weight loss reverts obesity-associated hypogonadotropic hypogonadism: a systematic review and meta-analysis. European Journal of Endocrinology, 2013. https://doi.org/10.1530/EJE-12-0955
- Nyamaah JA et al. Use of calculated free testosterone in men: advantages and limitations. Current Opinion in Endocrinology, Diabetes and Obesity, 2024. https://doi.org/10.1097/MED.0000000000000891
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