Both numbers come from the same blood draw, but they answer different questions. One describes supply. The other estimates availability. The protein that separates them is sex hormone binding globulin, written on most lab reports as SHBG.
Total testosterone is the concentration of all testosterone in a serum sample, counting the portion held tightly by SHBG, the portion held loosely by albumin, and the small unbound portion together. It is the number most laboratories report first, and it is usually drawn in the morning because output follows a daily rhythm. A single total value describes how much hormone is circulating, not how much of it reaches tissue.
That marker has its own dedicated page, so the detail on how the sample is collected, what typical and optimal values look like, and how to read one result in context sits with Testosterone Total. Everything below stays on the comparison itself, what the free number adds, when the two disagree, and what the difference is worth to you.
Free testosterone is the fraction of circulating testosterone that is not attached to any carrier protein. Most testosterone in blood travels bound. SHBG holds it with high affinity and releases it reluctantly, albumin holds it weakly but in large capacity, and only a small share is unbound at any given moment.
The reasoning behind the measurement is simple. Hormone that is not locked to a large carrier protein can leave the capillary and reach the androgen receptor inside a cell, so the unbound fraction is used as a stand in for how much androgen signal tissue is actually receiving 1. That is why two people with the same total result can feel and function differently, and why a clinician may want the free value before drawing any conclusion.
Some laboratories also report bioavailable testosterone, which adds the loosely albumin bound portion to the unbound portion on the reasoning that weak binding lets hormone dissociate readily as blood passes through tissue 1. Bioavailable and free testosterone usually move in the same direction, so if your report carries one of them, you are already looking at a binding adjusted number rather than raw supply.
One caution worth holding onto. Free testosterone is an estimate of exposure, not a measure of how well a receptor responds, and no single blood value describes how a person feels. It narrows the question. It does not close it.
Total testosterone is the broader, better standardised measurement, and free testosterone is the more interpretive one. The table below sets out where each is strong and where each falls short.
| Feature | Total testosterone | Free testosterone |
|---|---|---|
| What it measures | All testosterone in the sample, bound plus unbound | Only the fraction not bound to a carrier protein |
| Common units | ng/dL | pg/mL, sometimes ng/dL or a percentage of total |
| How it is obtained | Measured directly on serum | Measured after physical separation, or calculated from total testosterone, SHBG and albumin |
| Main strength | Widely available, and standardisation work supports comparison of results across laboratories 2 | Reflects differences in binding protein levels that the total value hides |
| Main limitation | Moves with SHBG even when production has not changed | Results depend heavily on the method used, so ranges are not interchangeable |
| Best used for | First line screening and tracking over time | Clarifying a borderline total, or any result where SHBG is unusually high or low |
Read together, they are complementary rather than competing. Ordering free testosterone without total testosterone and SHBG alongside it leaves you with a number that cannot be checked against anything.
SHBG is a glycoprotein made mainly in the liver that binds testosterone and other sex steroids in the bloodstream and governs how much hormone stays unbound 3. Think of it as the carrying capacity of your circulation. The more carriers there are, the smaller the share of hormone left floating free.
Because SHBG binds testosterone with high affinity, a change in SHBG concentration shifts the free fraction even when the testes or ovaries are producing exactly the same amount as before 3. High SHBG pulls the free value down while propping the total value up, since bound hormone still counts toward the total. Low SHBG does the reverse, flattering the free value while the total looks unimpressive.
Age is part of this picture too. Research suggests that SHBG concentrations tend to rise with age in men, which can leave an older man with a total result similar to a younger man while the free fraction sits lower 4. This is one of the clearest reasons a total value alone can be misleading in later decades.
SHBG is also informative in its own right. Studies indicate that SHBG concentrations differ across a range of clinical conditions, including metabolic, liver and thyroid disorders 5. So when a lab report shows an unexpected SHBG, the number is not just a correction factor for testosterone. It is a signal worth discussing on its own terms with a clinician.
This happens when SHBG is high, and it is the single most common reason the two numbers disagree. The observation is a total testosterone that looks unremarkable, a raised SHBG, and a free testosterone below the laboratory's range. What it may reflect is adequate production with reduced availability, because more of the hormone is held in bound form. What it is worth doing is bringing the full set of numbers, total, free, SHBG and albumin, to a clinician rather than acting on any one of them.
The mirror image is just as important. When SHBG is low, total testosterone can drift toward the bottom of the range while free testosterone stays reasonable, because a smaller share of hormone is bound. Acting on the total alone in that situation risks chasing a number that does not match tissue exposure.
There is a third pattern. Both numbers can be low together, which generally points toward reduced production rather than a binding effect, and that is a conversation for a clinician who can look at the rest of the hormonal picture. In each of these cases the useful question is not which number is right, but what combination of production and binding explains the pattern in front of you.
Total testosterone has published reference ranges that most laboratories report in similar terms, while free testosterone does not, because the free value depends on how it was obtained. For orientation, here is how the standard and optimal figures compare for total testosterone.
| Marker | Typical lab reference range | FitScript optimal range |
|---|---|---|
| Total testosterone, men | 264 to 1200 ng/dL | 500 to 900 ng/dL |
| Total testosterone, women | 8 to 120 ng/dL | 15 to 70 ng/dL |
These are typical lab reference ranges, and they vary by laboratory, age and sex. FitScript's optimal ranges are optimization targets used inside the membership, not diagnostic cut-offs, and a value outside them is not a diagnosis of anything.
Free testosterone is a different story. Because separation methods and calculation formulas do not produce identical figures, the only range that applies to your free result is the one printed by the laboratory that ran it. Standardisation and harmonisation work exists precisely because testosterone results have not always been comparable between methods 2. The practical rule is to compare a free testosterone value with its own report range, and to keep using the same laboratory and method when tracking change over time.
Free testosterone is either physically separated from its carrier proteins and then measured, or estimated by equation from total testosterone, SHBG and albumin. The method matters more here than for almost any other routine hormone marker.
| Method | What it does | Practical notes |
|---|---|---|
| Equilibrium dialysis with mass spectrometry | Separates unbound hormone across a membrane, then measures what crossed | Regarded as the reference approach for accuracy, and less widely available 6 |
| Ultrafiltration | Forces unbound hormone through a filter under pressure | Sensitive to temperature, dilution and handling technique 1 |
| Calculated free testosterone | Derives the free value from total testosterone, SHBG and albumin | Widely available and inexpensive, and dependent on the assumptions built into the formula 7 |
| Direct analog immunoassay | Single step assay reporting a free value without separation | Not regarded as a reliable way to assess the free fraction 1 |
For total testosterone, mass spectrometry based measurement has become the accuracy benchmark, particularly at the low concentrations seen in women 6. If your report does not say which method produced the free value, the ordering clinician or the laboratory can tell you, and it is a fair question to ask before you read much into a borderline number.
A calculated free testosterone is usually good enough to classify a result as high, middling or low, and it is not precise enough to hang a decision on a small change. The calculation takes your total testosterone, your SHBG and usually your albumin, then applies binding constants to estimate how much hormone is left unbound.
Research suggests that calculated free testosterone offers real practical advantages in men, including wide availability and reasonable agreement with separation methods, alongside limitations that come from the assumptions in the equations 7. Those assumptions include how tightly SHBG and albumin bind testosterone and whether binding behaves the same way in every person. Different published formulas therefore produce different numbers from the same blood sample.
Three practical consequences follow. First, a calculated free value is only as good as the inputs, so an inaccurate total testosterone or SHBG carries straight through into it. Second, comparing a calculated value from one laboratory with a dialysis value from another is not a like for like comparison. Third, when the decision is close, repeating the test, or asking whether a separation based measurement is available, is more useful than re-reading the same number.
Neither number alone settles a symptom question, though free testosterone often adds the most information once a total result lands near a boundary. Low energy, reduced libido, poor recovery, mood change and loss of muscle are all non-specific, and plenty of things besides testosterone produce them.
The sequence most clinicians work through is familiar. Start with a morning total testosterone. If it is clearly within range and symptoms are mild, the hormone is less likely to be the main story. If it is borderline or low, repeat it, and add SHBG and albumin so a free or bioavailable value can be interpreted alongside it. Accurate measurement of both total and free testosterone is described as central to assessing androgen status, which is why the combination is ordered rather than one number in isolation 6.
The honest summary is that total testosterone answers how much hormone there is, free testosterone answers how much is available, and symptoms answer neither. A licensed provider puts the three together.
SHBG responds to body composition, metabolic health, liver and thyroid function, age and diet, which is why it moves for reasons that have nothing to do with the testes or ovaries.
Diet contributes as well. Research suggests that nutritional factors, including the composition of the diet, influence circulating SHBG concentrations 8. That does not translate into a specific eating plan, and food choices are a conversation for a clinician or qualified professional who knows your history.
The free fraction itself tracks a similar set of influences. Pooled individual participant analyses report that calculated free testosterone in men is associated with sociodemographic, lifestyle and medical factors, age and body size among them 9.
Some medicines also change SHBG or testosterone values, so ask your prescriber whether anything you take could be affecting your results before you interpret them. The point of knowing all this is not to self-correct a number. It is to recognise that a single out of range SHBG has several possible explanations, and the explanation changes what the testosterone values mean.
The principle is identical in women, but the measurement problem is harder, because testosterone circulates at far lower concentrations. At those levels, assay accuracy becomes the limiting factor, and mass spectrometry based measurement is emphasised for reliable assessment of androgen status in women 6.
SHBG tends to sit higher in women than in men, so the unbound share of an already small total is smaller still. That makes the free or calculated free value, read alongside SHBG, more informative than a total figure on its own when androgen related symptoms are being assessed. It also makes low quality assays more misleading, since an imprecise total feeds directly into any calculated free result.
As in men, a single value does not define anything. Androgen levels in women shift across the menstrual cycle and with age, and some medicines change SHBG, so ask your prescriber about anything you take before reading too much into a result. Patterns across repeated, well timed tests are worth more than one measurement, and interpretation belongs with a clinician who knows the full clinical picture.
SHBG and albumin come first, because without them a free value cannot be calculated or sanity checked. After that, the useful companions depend on the question being asked.
Luteinising hormone and follicle stimulating hormone help indicate whether the signal from the brain to the gonads is intact or exaggerated. Estradiol matters because testosterone converts into it, and prolactin is often included when the pituitary is in question. A complete blood count gives context on red cell parameters. Thyroid markers and liver enzymes round out the common additions, since both organs influence SHBG production.
This is the logic behind testing broadly rather than one marker at a time. FitScript's membership covers 150+ biomarkers tested per year, and Atlas, FitScript's AI health advisor, reads those markers together rather than in isolation, which is where a free versus total mismatch usually becomes obvious.
Testosterone varies through the day and from week to week, so one value is a snapshot rather than a verdict. Levels are generally highest in the morning in men, which is why morning sampling is standard, and why comparing a morning draw with an afternoon draw tells you very little.
Acute illness, a run of poor sleep, heavy training in the preceding day or two, significant weight change and major stress can all move a result temporarily. Laboratory factors matter as well, since methods differ in accuracy and standardisation efforts exist to make results comparable between laboratories 2. Changing laboratory mid investigation introduces a difference that has nothing to do with your physiology.
For free testosterone specifically, an error in SHBG or albumin propagates into the calculated value, and sample handling influences separation based methods 1. The practical response to any surprising result is the same. Repeat it, under the same conditions, at the same laboratory, before anyone draws conclusions from it.
Bring any result that is outside the laboratory's range, or any persistent symptom pattern, to a clinician, and do it promptly when the value sits well outside the range or symptoms are affecting daily function. A result that is only mildly suboptimal is still worth raising, because the interpretation depends on your age, symptoms, medications and the rest of your panel.
Useful preparation makes the appointment better. Take the full report rather than one line from it, note the time of the draw, list anything you take regularly, and flag any recent illness, weight change or sleep disruption. If the free value was calculated rather than separated, say so, because it changes how much weight the number should carry.
This article is educational and is not medical advice. Discuss your results and any treatment with a clinician or licensed provider, who can order confirmatory testing and decide what, if anything, should follow. Where a prescription option comes up, a licensed provider evaluates whether it is appropriate for you, and treatment decisions belong to a licensed provider. Never ignore an out of range result, and never stop or start anything on the strength of a single number.
For related reading, see the guide to low testosterone in men, the explainer on how testosterone therapy works, and the practical walkthrough of how to test testosterone levels.
Total testosterone is the usual starting point because it is widely available and well standardised. Free testosterone is most useful as a second number, especially when the total sits near a boundary or when SHBG is unusually high or low. Ordering SHBG and albumin at the same time lets a clinician interpret both figures together.
Yes, and it is the most common reason the two numbers disagree. A high SHBG holds more of the hormone in bound form, which keeps the total looking fine while the unbound share falls. The pattern is worth discussing with a clinician rather than acting on either number alone.
SHBG is a carrier protein made mainly in the liver that binds testosterone in the bloodstream. The higher it is, the smaller the free fraction, even when production has not changed. It is also needed as an input for any calculated free testosterone value.
Because laboratories use different methods, including physical separation techniques and different calculation formulas, and those methods do not produce identical numbers. That is why a free value should be read against the range printed on its own report. Sticking to one laboratory and one method makes tracking over time far more meaningful.
For most purposes it is a reasonable way to tell whether the free fraction is high, middling or low. It is less suited to judging small changes, because it relies on assumptions in the formula and on the accuracy of the total testosterone, SHBG and albumin that feed into it. When a decision is close, your clinician may repeat the test or ask about a separation based measurement.
Morning sampling is standard for men because testosterone follows a daily rhythm and is generally highest earlier in the day. Keeping the time consistent between tests matters as much as the time itself. Mention the draw time when you review the report, since it changes how the result reads.
No. A single value is a snapshot, and illness, poor sleep, heavy training or a late draw can all move it. Confirmation usually means repeating the test under similar conditions, and a clinician decides what the pattern means and what should follow.
Often yes, because women circulate much lower amounts of testosterone and tend to have higher SHBG, so the total figure alone can be hard to interpret. Assay accuracy matters a great deal at those low concentrations. A clinician decides which version of the test is appropriate and how to read it alongside the rest of the panel.