Free Testosterone vs Total Testosterone: The Difference
Total testosterone is all the testosterone in your blood, bound and unbound. Free testosterone is only the small unbound fraction, about 1 to 4% of the total, which under the free hormone hypothesis is the part that tissues can use.[1] Most standard panels report total testosterone, and guidelines add free testosterone when the total is near the lower limit of normal or SHBG may be abnormal.[2]
You get your blood work done. Your doctor says your testosterone is normal. But you still feel sluggish, your libido is lower than it should be, and your recovery from training has deteriorated. Can testosterone be “normal” while you have symptoms?
Sometimes the explanation lies in the difference between total and free testosterone. These are different measurements, and many standard panels only report total testosterone. (Fatigue and low libido have many possible causes, so a normal or low result is a conversation for a doctor, not a self-diagnosis.) Knowing where to get tested matters too: see our guide on home testosterone test kits vs. lab testing.
Key takeaways
- Only about 1 to 4% of circulating testosterone is unbound (free); the rest is bound to SHBG and albumin.[1]
- The Endocrine Society recommends diagnosing low testosterone on repeated fasting morning total testosterone, and adding free testosterone when total is near the lower limit or SHBG may be abnormal.[2]
- Calculated free testosterone agrees closely with equilibrium dialysis, while direct analog free testosterone assays can be unreliable.[3][1]
- Time of day and food matter: testosterone was 20 to 25% lower at 4 pm than 8 am in men aged 30 to 40,[4] and a 75 g glucose drink lowered it by about 25%.[5]
Free Testosterone vs Total Testosterone at a Glance
| Total testosterone | Free testosterone | |
|---|---|---|
| What it measures | All testosterone in the blood: bound to SHBG and albumin, plus unbound.[1] | Only the unbound fraction, roughly 1 to 4% of the total.[1] |
| Role in guidelines | The first test: fasting morning total testosterone, repeated to confirm.[2] | A second look when total is near the lower limit of normal or SHBG may be abnormal.[2] |
| How it is obtained | A standard blood test, reported by most panels. | Equilibrium dialysis (reference), calculation from total T, SHBG and albumin, or a direct analog immunoassay.[3] |
| Reliability caveat | Results vary between assays, so compare only with your own lab's range.[8] | Direct analog assays can be unreliable, while calculated values agreed closely with equilibrium dialysis.[3][1] |
| Affected by SHBG? | It does not show how much is bound. Two men with the same total can have different SHBG. | Yes. At the same total, higher SHBG means less is free (see the worked example below). |
What Total Testosterone Actually Measures
When a standard blood test reports your testosterone level, it is measuring total testosterone: the sum of all testosterone in your blood, bound or not. Most circulating testosterone is bound to carrier proteins. It is bound tightly to sex hormone-binding globulin (SHBG) and weakly to albumin, with smaller amounts on other proteins, and only about 1 to 4% is unbound.[1]
According to the free hormone hypothesis, only that unbound fraction is biologically active in target tissues. The same review that reports these figures notes that this hypothesis has been debated extensively and that the prevailing binding models are not fully supported by experimental data.[1] In practice, clinicians use free testosterone as an extra piece of information when total testosterone and symptoms do not line up, not as a replacement for total testosterone.
The combination of free and albumin-bound testosterone is sometimes called bioavailable testosterone, because albumin binds testosterone weakly.[1]
Why Free Testosterone Can Matter
Think of total testosterone as your gross salary and free testosterone as your take-home pay. Two men can have the same total testosterone but different SHBG, and therefore different amounts of free testosterone. Here is an illustrative calculation (our own worked example, using the Vermeulen equation and an assumed albumin of 43 g/L):
✓ Total T 550 ng/dL + SHBG 35 nmol/L
- •Calculated free T: about 11 ng/dL
- •Free fraction: about 2% of total
- •Same total testosterone as the other example
- •Looks identical on a total-only test
✗ Total T 550 ng/dL + SHBG 70 nmol/L
- •Calculated free T: about 7 ng/dL
- •Free fraction: about 1.3% of total
- •Same total testosterone as the other example
- •Looks identical on a total-only test
The numbers show why a total-only panel can hide differences. They do not tell you whether either man has symptoms or a medical problem. That needs a clinician, and the lab's own reference ranges.
What the guidelines say
The Endocrine Society guideline recommends diagnosing hypogonadism only in men with symptoms and signs of testosterone deficiency and consistently low testosterone, starting with fasting morning total testosterone, confirmed by repeating the measurement. In men whose total testosterone is near the lower limit of normal, or who have a condition that alters SHBG, it recommends obtaining free testosterone by equilibrium dialysis or an accurate formula.[2]
What Raises and Lowers SHBG
SHBG is made mainly in the liver and is influenced by hormones, metabolism and age. These associations are well described, but the size of the effect varies between people:
Factors associated with higher SHBG (which can lower free T relative to total T):
- Aging: in the Massachusetts Male Aging Study (men aged 40 to 70), SHBG rose about 1.6% per year, while free testosterone declined about 2% per year versus about 0.8% per year for total testosterone in cross-sectional analysis.[6] That is part of why free testosterone tends to fall faster than total with age. See our guide to testosterone levels by age.
- Hyperthyroidism: extremely high SHBG is seen in hyperthyroidism.[3]
- Liver disease and some medications: including some anticonvulsants. Ask your doctor or pharmacist if you take regular medication.
- Weight loss through low-calorie diets: a meta-analysis found that weight loss raised both bound and unbound testosterone in men with obesity, so the net effect on free testosterone is usually positive in that group.[7]
Factors associated with lower SHBG: obesity and insulin resistance, type 2 diabetes, and androgen (anabolic steroid) use. This is why men with obesity often have low SHBG and low total testosterone together, which is covered in our article on the body fat and testosterone connection.
How Free Testosterone Is Measured
Free testosterone can be measured or estimated in three main ways: equilibrium dialysis (the reference approach), calculation from total testosterone, SHBG and albumin (for example the Vermeulen equation), or a direct analog immunoassay.
In a comparison across a wide range of SHBG levels, calculated free testosterone was almost identical to equilibrium dialysis values (except in pregnancy), while direct analog assay values were only a fraction of them and varied with SHBG.[3] A later review likewise reported that direct immunoassays for free testosterone lack accuracy and significantly underestimate values compared with equilibrium dialysis, noting that the Endocrine Society recommends against using them.[1] If your report lists total testosterone, SHBG and albumin, you can estimate free testosterone yourself with our free testosterone calculator, which uses the Vermeulen equation. It is an educational estimate, not a diagnosis.
Direct Free T Results Can Mislead
If your report shows a “free testosterone” value from a direct immunoassay, it may not be accurate, especially if your SHBG is unusual. Ask your doctor whether it was measured by equilibrium dialysis or calculated, and only compare your result to the reference range printed on that same report. Because methods differ, numbers quoted online often cannot be compared with yours.
Reference Ranges: What We Can and Cannot Say
For total testosterone, a harmonized analysis of 9,054 community-dwelling men in four cohort studies found a healthy non-obese range of 264 to 916 ng/dL (2.5th to 97.5th percentile) for men aged 19 to 39. The authors also found that a large share of the variation between studies came from differences between assays.[8]
For free testosterone and SHBG we do not give age-by-age “optimal” ranges. Free testosterone numbers depend heavily on the method used, and we are not aware of an agreed set of optimal targets backed by outcome studies. Use the ranges on your own lab report and have a clinician interpret them alongside your symptoms, age and health.
How to Get an Accurate Testosterone Assessment
Getting Reliable Results
Ask your doctor which tests make sense
Total testosterone is the usual starting test. If it is near the lower limit of normal, or a condition that alters SHBG is suspected, ask about SHBG, albumin and free testosterone by equilibrium dialysis or calculation. A doctor may also check LH and FSH to understand why levels are low.
Test in the morning, fasted
Testosterone is higher in the morning: in men aged 30 to 40 levels were 20 to 25% lower at 4 pm than at 8 am. Food also matters, since a 75 g oral glucose load lowered testosterone by about 25% in one study. Guidelines recommend fasting morning samples.
Repeat the test before drawing conclusions
A single draw is a snapshot influenced by sleep, stress, illness and training. The Endocrine Society recommends confirming a low result by repeating the fasting morning measurement.
Know how free T was obtained
Check whether free testosterone was measured by equilibrium dialysis, calculated, or from a direct analog assay, which can underestimate it.
Interpret results in clinical context
Numbers alone do not determine whether treatment is needed. Your doctor will weigh symptoms, age, other health conditions and your goals.
For a more detailed walkthrough, see our complete guide to testosterone testing. If you have persistent symptoms, our guide on when to see a doctor explains what to expect.
Lifestyle Factors That Support Healthy Levels
Free testosterone depends on total testosterone production and on SHBG, so the foundations are the same as for overall hormone health: enough sleep, regular resistance training, stress management, a healthy body weight, and a nutritious diet including adequate zinc and magnesium and vitamin D. In men with overweight or obesity, weight loss is associated with higher testosterone.[7]
Can You Change SHBG With Supplements?
If your labs show high SHBG, the first step is to work out why, since causes include age, thyroid or liver conditions and medications. Ask your doctor to review the panel before assuming a supplement is the answer. The evidence for supplements is thin:
- Boron. In a tiny study of 8 healthy men, a 10 mg dose of boron was followed by a decrease in SHBG within 6 hours, and after 7 days of 10 mg daily, free testosterone was higher and estradiol lower. With 8 participants and no long-term follow-up, treat it as preliminary.[9] Details in our boron guide.
- Magnesium. A small study in which men took 10 mg of magnesium per kg of body weight daily for four weeks reported higher free and total testosterone, especially in those who exercised.[10] That dose is far above the commonly cited 350 mg per day upper limit for supplemental magnesium, so do not copy it. See zinc and magnesium.
- Vitamin D. Results are mixed. A randomized trial in overweight men with low vitamin D on a weight-loss program found that 3,332 IU daily for one year raised total, bioactive and free testosterone,[11] but a later 12-week trial in healthy men with 20,000 IU per week found no effect on total testosterone.[12] Get your level checked before supplementing; see vitamin D and testosterone.
Supplements can interact with medicines and conditions, so speak with a doctor before starting any. SHBG is one piece of a larger picture: sleep, stress, training and nutrition come first.
The Bottom Line
Total testosterone is the standard first test, and free testosterone is a useful second look when total is borderline or SHBG may be unusual. How free testosterone is measured matters as much as the number. If you have symptoms, the right move is to see a doctor, ask which tests and methods are being used, and repeat testing before drawing conclusions.
Related Reading
- How to Increase Testosterone Naturally : lifestyle levers for healthy hormone levels.
- Signs of Low Testosterone : symptoms worth discussing with a doctor.
How Thrive Helps
Thrive helps you build the habits that support healthy hormones: sleep, training, nutrition and stress management. Track your daily habits, monitor progress over time and see which habits have stronger research behind them. Thrive is a wellness and habit tracking app, not a medical device, and cannot diagnose or treat low testosterone. Try Thrive on the App Store.
References
- Goldman AL, Bhasin S, Wu FCW, Krishna M, Matsumoto AM, Jasuja R. A reappraisal of testosterone's binding in circulation: physiological and clinical implications. Endocrine Reviews. 2017. doi.org/10.1210/er.2017-00025
- Bhasin S, Brito JP, Cunningham GR, et al.. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism. 2018. doi.org/10.1210/jc.2018-00229
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology & Metabolism. 1999. doi.org/10.1210/jcem.84.10.6079
- Brambilla DJ, Matsumoto AM, Araujo AB, McKinlay JB. The effect of diurnal variation on clinical measurement of serum testosterone and other sex hormone levels in men. Journal of Clinical Endocrinology & Metabolism. 2009. doi.org/10.1210/jc.2008-1902
- Caronia LM, Dwyer AA, Hayden D, et al.. Abrupt decrease in serum testosterone levels after an oral glucose load in men: implications for screening for hypogonadism. Clinical Endocrinology. 2013. doi.org/10.1111/j.1365-2265.2012.04486.x
- Feldman HA, Longcope C, Derby CA, 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 & Metabolism. 2002. doi.org/10.1210/jcem.87.2.8201
- Corona G, Rastrelli G, Monami M, et al.. Body weight loss reverts obesity-associated hypogonadotropic hypogonadism: a systematic review and meta-analysis. European Journal of Endocrinology. 2013. doi.org/10.1530/EJE-12-0955
- Travison TG, Vesper HW, Orwoll E, 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 & Metabolism. 2017. doi.org/10.1210/jc.2016-2935
- Naghii MR, Mofid M, Asgari AR, Hedayati M, Daneshpour MS. Comparative effects of daily and weekly boron supplementation on plasma steroid hormones and proinflammatory cytokines. Journal of Trace Elements in Medicine and Biology. 2011. doi.org/10.1016/j.jtemb.2010.10.001
- Cinar V, Polat Y, Baltaci AK, Mogulkoc R. Effects of magnesium supplementation on testosterone levels of athletes and sedentary subjects at rest and after exhaustion. Biological Trace Element Research. 2011. doi.org/10.1007/s12011-010-8676-3
- Pilz S, Frisch S, Koertke H, et al.. Effect of vitamin D supplementation on testosterone levels in men. Hormone and Metabolic Research. 2011. doi.org/10.1055/s-0030-1269854
- Lerchbaum E, Pilz S, Trummer C, et al.. Vitamin D and testosterone in healthy men: a randomized controlled trial. Journal of Clinical Endocrinology & Metabolism. 2017. doi.org/10.1210/jc.2017-01428