What is the difference between free vs total testosterone?
The difference comes down to proteins. Total testosterone counts every molecule in your blood. Free testosterone counts only the small share that is not attached to a protein, about 2%.
Around 40% is held tightly by SHBG (sex hormone-binding globulin). The rest is held loosely by albumin and other proteins, according to the label for one enanthate product.
Total is the standard first test. Guidelines add free testosterone in specific cases, mainly when the total is borderline or SHBG is likely to be unusual. Free testosterone is one of the tests in our guide to which labs to monitor on TRT, and this page covers it in full.
Why is most testosterone bound to proteins?
Testosterone does not dissolve well in blood, so it travels on carrier proteins. The two that matter are:
- SHBG, made by the liver, which grips testosterone tightly.
- Albumin, the most common blood protein, which holds it loosely and lets go easily.
The exact split depends on the source and the method. The Endocrine Society says only 2.0% to 4.0% of testosterone in the blood is free. In a study that measured free testosterone directly in 146 men, the median was 1.5% of the total.
The free part and the albumin-bound part together are called bioavailable testosterone. Both are thought to reach tissues. The SHBG-bound part is held too tightly to count.
Why can two men with the same total testosterone have different free testosterone?
Because their SHBG differs. With the same total, a man with high SHBG has more testosterone locked up and less free. A man with low SHBG has more free.
This is the main reason free testosterone exists as a test. Ageing, liver disease and an overactive thyroid push SHBG up. Obesity, diabetes and an underactive thyroid push it down.
Our SHBG page covers what moves it and what it means on TRT.
How much does SHBG change free testosterone in practice?
More than most people expect. The table below runs four made-up results through the formula most calculators use, called the Vermeulen equation. Albumin is held at a typical 4.3 g/dL.
| Total testosterone | SHBG | Calculated free testosterone | Share free |
|---|---|---|---|
| 450 ng/dL | 20 nmol/L | about 120 pg/mL | about 2.7% |
| 450 ng/dL | 60 nmol/L | about 62 pg/mL | about 1.4% |
| 350 ng/dL | 20 nmol/L | about 91 pg/mL | about 2.6% |
| 350 ng/dL | 60 nmol/L | about 47 pg/mL | about 1.3% |
Illustration only, calculated by TRTracker with the Vermeulen equation; not reference values.
With the same total of 450 ng/dL, tripling SHBG roughly halves the free testosterone. A man with a total of 350 and low SHBG ends up with more free testosterone than a man with 450 and high SHBG. That is what the guideline means when it says total testosterone alone can mislead.
When do guidelines recommend checking free testosterone?
The Endocrine Society starts with a fasting morning total testosterone. It adds free testosterone in two cases:
- Total testosterone is at or near the lower limit of normal, so the total alone cannot settle it.
- A condition is likely to change SHBG, which makes the total misleading.
It says to measure free testosterone by equilibrium dialysis done under standard conditions. If that is not available, it says to estimate it with an accurate formula.
The AUA also bases diagnosis on total testosterone and does not use free testosterone as the main test. It sees a role for free testosterone when the total is borderline, roughly 230 to 317 ng/dL. It also sees a role in men with strong symptoms despite a low-normal total.
What are the three ways to measure free testosterone?
A result labelled “free testosterone” can come from three very different tests. This matters more than most people realise.
| Method | How it works | How reliable | Example range (adult men) |
|---|---|---|---|
| Equilibrium dialysis | Separates free from bound testosterone, then measures it, often by mass spectrometry | Reference method; slower and more expensive | 52–280 pg/mL (one large US lab) |
| Calculated | An equation using total testosterone, SHBG and albumin | Close to dialysis in most men | Lower limit 70 pg/mL in healthy young men (one US study) |
| Direct (analog) immunoassay | A labelled look-alike competes with testosterone for an antibody | Not reliable; tracks total, not free | 6.6–26.5 pg/mL, by age (same US lab) |
Equilibrium dialysis
Dialysis physically separates the free part before measuring it. It is the method other tests are judged against. At one large US lab, free testosterone by dialysis is 1.5% to 3.2% of the total, and the adult male range is 52 to 280 pg/mL.
Calculated free testosterone
Your lab or prescriber plugs total testosterone, SHBG and albumin into an equation. In the study that set the standard, calculated and dialysis values were almost the same in every condition tested except pregnancy.
A later study of 146 men found the Vermeulen equation reads slightly high, with a median ratio of 1.19 to dialysis. It still called it the most robust estimate.
Not everyone agrees. A detailed review argues that the binding models behind the equations are probably inaccurate. A fair summary: calculated free testosterone is a good estimate, not an exact measurement.
Direct (analog) immunoassay
This is the cheap test on many lab menus, often called “free testosterone, direct”. An immunoassay is a test that uses antibodies to detect a substance. In the study that set the standard, direct results were only a fraction of the dialysis and calculated values, and that fraction varied with SHBG.
A later lab study found the analog test did not detect free testosterone at all. It said the test should not be used for this purpose.
The Endocrine Society agrees. It says clinicians should not use direct analog free testosterone tests, because they are inaccurate.
Is the free androgen index the same as free testosterone?
No. The free androgen index (FAI) divides total testosterone by SHBG and multiplies by 100. It is quick, but it is a ratio, not a measurement of free testosterone.
The study that compared the methods found that the FAI behaves differently as SHBG changes. It concluded that neither the direct test nor the FAI is a reliable guide to bioavailable testosterone.
If your report shows an FAI, treat it as a rough pointer. Ask your prescriber whether a calculated or dialysis free testosterone would be more useful.
What is bioavailable testosterone, and is it measured?
Bioavailable testosterone is the free part plus the part loosely bound to albumin. In effect, it is everything not held by SHBG. Because albumin lets go easily, this pool is thought to be available to tissues.
It is measured with a separate lab method that removes the SHBG-bound part first, and it is rarely ordered. Some calculators also report an estimated bioavailable value next to free testosterone.
It has its own, much higher reference range. So do not compare it with a free testosterone range.
What is a normal free testosterone level?
There is no single number, because each method has its own scale. Look at your report and find two things: the method (dialysis, calculated or direct) and the range printed next to your result.
For calculated free testosterone, researchers have proposed two reference points:
- 70 pg/mL: the 2.5th percentile in 456 healthy, non-obese men aged 19 to 40 in a large US heart study. That means 2.5 in every 100 of these men were below it.
- 64 pg/mL (220 pmol/L): a European study of 3,369 men aged 40 to 79 used this cutoff. Below it, with a low total and at least three sexual symptoms, it defined late-onset hypogonadism (low testosterone that starts in middle or later life).
If your result is in single digits or the low teens of pg/mL, it almost certainly came from a direct test. Direct-test ranges at one US lab run from 6.6 to 26.5 pg/mL, depending on age. Comparing that number with a dialysis range would make anyone look severely low.
Does free testosterone fall with age?
Yes, and faster than total testosterone. In one long-running US study, total testosterone fell about 0.8% a year across age groups. Free and albumin-bound testosterone each fell about 2% a year, and SHBG rose about 1.6% a year.
The rising SHBG explains much of the gap. That is why an older man can have a total testosterone that looks fine and a free testosterone that does not.
It is also why the Endocrine Society lists conditions that change SHBG, including ageing, as a reason to check free testosterone. For age-based total testosterone ranges, see our page on normal testosterone levels by age.
What happens to free testosterone on TRT?
It rises, along with total testosterone. In the Testosterone Trials, older men on testosterone gel had higher total and free testosterone after 12 months, measured by dialysis. The percent free also rose, while SHBG did not change.
The same analysis found that small changes in estradiol, a form of estrogen, moved the free share more than changes in testosterone itself. So on treatment, you cannot work out free testosterone from the total with a fixed percentage. Our estradiol page explains why that hormone rises on TRT.
Guidelines monitor total testosterone and hematocrit (the share of your blood made of red cells) during the first year, not free testosterone. Some prescribers add free testosterone when SHBG is unusual or symptoms do not fit the total. Whether to test it, and what to do with the result, is their call.
How does draw timing change free testosterone on TRT?
Free testosterone rises and falls with the total. On injections, both peak a few days after a shot and are lowest just before the next one. A free testosterone drawn at peak and one drawn at trough can look like two different men.
Our guide to trough vs peak testosterone explains what each number means.
If your free testosterone is calculated, the total and the SHBG in the equation should come from the same blood sample. Mixing a peak total with an SHBG from another month gives a number that does not describe any real moment. When you use an online calculator, take both values from one report and note the time of the draw.
Use the same timing for every draw so results compare. The blood test timing tool and our TRT blood work schedule cover how to time a draw. The testosterone level calculator shows how your current schedule shapes the curve between injections.
What should you ask your prescriber about free testosterone?
- Which method did the lab use: dialysis, calculated or direct?
- Is the range on my report for that method?
- Would free testosterone change anything, given my total and my SHBG?
- If it is calculated, which total testosterone and SHBG went into it?
- Should we repeat it at the same time in my injection cycle?
How can you track free testosterone over time?
Log each free testosterone with its method, units and range, next to the total and SHBG from the same draw. Switching methods between draws makes the numbers jump for no real reason.
In TRTracker, each result sits against the range your lab printed. A method change then shows up as a change of range, not a mystery drop. Our TRT blood work guide lists every marker commonly checked on TRT, including hematocrit, the one prescribers watch most closely.