Testosterone: Free, Bioavailable, and Total
This test reports three related measurements from a single blood sample: total testosterone, the free fraction, and the bioavailable fraction. Together they describe not just how much hormone is present but how much of it tissues can actually use.
About this test
Testosterone does not travel through the bloodstream on its own. Most of it is held tightly by sex hormone binding globulin, a smaller share rides loosely on albumin, and only a small fraction circulates unbound.
That distinction is the whole point of ordering all three. Binding globulin varies a great deal between people and shifts with age, thyroid activity, liver health, body weight, and certain medications.
Who might consider this test?
- Men often reach this test after a total testosterone result did not line up with how they feel
- symptoms that suggest low androgen activity paired with an unremarkable total measurement, or a low total alongside few symptoms.
- Adding the free and bioavailable fractions is the usual next step in sorting that out with a clinician.
Test details
| Collection | Quest patient service center |
|---|
Quest resources
Additional details
Total, free, and bioavailable — what each one counts
Total is the whole pool: tightly bound, loosely bound, and free combined. Free is the small unbound share that moves freely into tissues. Bioavailable adds the albumin-bound share to the free share, because albumin releases its cargo readily enough that tissues can generally access it.
Thinking of it as supply versus access helps. Total describes supply. Free and bioavailable describe access. When the two tell different stories, the reason usually lies with the binding proteins in between.
Why binding proteins change the picture
Sex hormone binding globulin is not a fixed quantity. It tends to rise with age and with thyroid activity, and it tends to fall with obesity and insulin resistance. Because it grips testosterone tightly, more binding globulin means a larger share of the total is unavailable, and less means the opposite.
This is why two people with similar total measurements can be in genuinely different situations, and why a clinician may want the fractions before saying anything about what a result means.
How the measurements are produced
Laboratories can determine the free and bioavailable fractions by direct methods such as equilibrium dialysis or by calculation from total testosterone, binding globulin, and albumin. The approaches do not always agree perfectly, so comparing results across laboratories or across methods can be misleading.
If you are following your levels over time, it is worth keeping your testing consistent and mentioning any change in method to the clinician reviewing your results, so trends mean what they appear to mean.
Ordering and what it costs
If this specific combination is not currently orderable online, you can request it and our team will follow up with availability and pricing rather than leaving you to guess.
This test isn't part of the Celio Health store at the moment, so it can't be ordered or requested here. The test menu lists everything that can.
Laboratory results should always be interpreted by a licensed clinician who knows your health history. This page is information about a test, not medical advice.
Frequently asked questions
What is the difference between free and bioavailable testosterone?
Free counts only the unbound hormone. Bioavailable counts the free portion plus the portion loosely attached to albumin, since albumin releases testosterone readily enough that tissues can generally access it.
Do I need all three measurements?
Not always. Many questions are answered by total testosterone, sometimes with sex hormone binding globulin alongside it. The fuller set is most useful when binding proteins are likely to be unusual or when symptoms and a total result disagree.
When should the blood be drawn?
A morning collection is usually preferred, because testosterone follows a daily rhythm and tends to be highest earlier in the day. Consistent timing also makes results easier to compare over time.
Why do my results differ from a previous lab's?
Free and bioavailable testosterone can be measured directly or calculated, and different laboratories use different approaches. Mention any change in laboratory or method to the clinician interpreting your results.