The short answer
Total testosterone measures all the testosterone in your blood, including the portion bound to proteins. Free testosterone is the small fraction not bound to sex hormone binding globulin, and is the number that becomes useful when SHBG is unusually high or low. Total testosterone is the standard first test; free testosterone is added in specific situations, and neither is interpreted without symptoms, timing and repeat measurement.
Two men can have identical total testosterone results and very different hormone situations, because most testosterone in blood is bound to carrier proteins and unavailable to tissues. Understanding what each test measures explains why clinicians order them in a particular sequence, and why a printed number rarely tells the whole story.
Key takeaways
- Total testosterone includes protein-bound hormone; free testosterone is the unbound fraction.
- SHBG determines the relationship between them, and common conditions after 40 shift it.
- Total testosterone is the standard first test; free testosterone is added selectively.
- Morning sampling and repeat measurement are part of a valid result.
- Assay and laboratory differences mean results are not always directly comparable.
Bound, free and everything in between
Most circulating testosterone is attached to sex hormone binding globulin (SHBG), which holds it tightly. A further large share is loosely bound to albumin, and only a small percentage circulates completely unbound.
Free testosterone refers to that unbound fraction. Bioavailable testosterone refers to the free portion plus the loosely bound albumin portion, on the basis that albumin releases hormone readily.
Because SHBG concentration determines how much of the total is held back, anything that shifts SHBG shifts the relationship between total and free testosterone.
Side by side
| Total testosterone | Free testosterone | |
|---|---|---|
| What it measures | All testosterone in the sample, bound and unbound | The unbound fraction only |
| Usual role | Standard first-line test | Added when total is borderline or SHBG is likely abnormal |
| How it is obtained | Direct laboratory measurement, ideally by mass spectrometry | Calculated from total testosterone, SHBG and albumin, or measured by equilibrium dialysis |
| Main weakness | Can mislead when SHBG is high or low | Direct immunoassay versions are unreliable; calculated values depend on the equation used |
| Best conditions | Morning sample, fasting where the laboratory requests it, repeated on a second morning | Same sample requirements, plus a reliable SHBG result |
What raises and lowers SHBG
SHBG is the reason free testosterone exists as a separate question. Several common situations after 40 move it in opposite directions.
- Higher SHBG is associated with older age, hyperthyroidism, liver disease, some anti-seizure medicines and marked weight loss.
- Lower SHBG is associated with obesity, type 2 diabetes and insulin resistance, hypothyroidism, nephrotic syndrome and glucocorticoid or androgen use.
- When SHBG is low, total testosterone can read low while free testosterone is adequate. When SHBG is high, total can read reassuring while free is low.
Why the same man gets different numbers
Testosterone follows a daily rhythm, with higher concentrations in the morning, which is why guidelines specify morning sampling.
Biological variation between days is substantial, and acute illness, sleep loss, heavy training and severe calorie restriction all depress results temporarily. This is why a single measurement is not treated as definitive.
Assays also vary. Results from different laboratories, or from immunoassay versus mass spectrometry, are not always interchangeable, so comparing results across laboratories can create differences that are technical rather than biological.
Putting the results in context
Testosterone results are interpreted alongside symptoms, physical findings and often additional tests such as LH, FSH and prolactin, which help distinguish problems originating in the testes from those originating in the pituitary or hypothalamus.
A number close to the lower reference limit is not a diagnosis. Nor is a mid-range number a guarantee that hormones play no part in how you feel. The clinical question is whether a consistent pattern of symptoms and repeated measurements fits together.
Commonly confused
“Free testosterone is always the more accurate test.”
It is more informative in specific situations. Calculated free testosterone depends on the accuracy of the total testosterone and SHBG results feeding into it.
“There is a single normal range by age.”
Reference ranges vary by laboratory and assay, and simplistic age tables circulating online do not reflect how results are interpreted clinically.
“One result is enough to act on.”
Guidelines call for confirming a low result on a separate morning before making decisions.
Questions to raise with a healthcare professional
- Was this drawn in the morning, and should we repeat it?
- Does my SHBG suggest that free testosterone would add information?
- Which assay did the laboratory use, and are my results comparable across draws?
- Do my symptoms match the pattern these results suggest?
- Would LH, FSH or prolactin help identify the cause?
References
- Endocrine Society. Testosterone Therapy in Men With Hypogonadism: clinical practice guideline
- MedlinePlus (National Library of Medicine). Testosterone levels test
- American Urological Association. Testosterone Deficiency guideline
How this article is reviewed
Aldrick articles are written from published guidance and peer-reviewed research, checked against our evidence standards and re-reviewed when guidance changes. Spotted something inaccurate or out of date? Tell us and we will correct it and update the review date.
This article is educational. It does not diagnose conditions or replace evaluation by a qualified clinician.