Testosterone: Free, Bioavailable, and Total

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. Total testosterone counts all of it. Free counts only the unbound portion. Bioavailable counts the free portion plus the loosely albumin-bound portion, on the reasoning that hormone held loosely can still reach tissues while hormone bound tightly generally cannot.

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. When binding protein is unusual, a total measurement can give a misleading impression of how much hormone is actually working, and the free and bioavailable numbers fill that gap.

The sample is a routine blood draw at a Quest patient service center. Clinicians generally ask for a morning collection, since testosterone follows a daily rhythm and tends to run highest earlier in the day. Celio Health arranges the physician's order for the tests it offers, so there is no separate doctor's visit to schedule and no insurance is billed at any point.

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.

It is also ordered when something is known to affect binding proteins: thyroid conditions, liver disease, significant weight change, aging, or medications that influence hormone binding. In those situations the extra detail is not a luxury, it is what makes the result interpretable. People without any of these factors often find that a total measurement, sometimes with binding globulin alongside it, answers their question at lower cost.

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.

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.

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.

The order box on this page shows the options available for these markers, along with what each one costs before you commit — Celio Health prices are cash prices with the physician's order included, and no insurance is billed. 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.

Results will be emailed securely to you as a PDF.