What is bioavailable testosterone?

Bioavailable testosterone counts the testosterone in your blood that your body can actually use. This includes free testosterone (unbound hormone) and testosterone bound loosely to albumin, a major blood protein. Together, these forms represent the active hormone available to bind receptors in muscle, bone, brain, and other tissues.

Standard lab tests measure total testosterone, which adds free testosterone to albumin-bound testosterone to SHBG-bound testosterone (sex hormone-binding globulin). SHBG binds testosterone tightly, making it unavailable for immediate use. A man might have normal total testosterone but low bioavailable testosterone if SHBG levels are elevated due to aging, liver disease, or certain medications. Conversely, high SHBG can mask low total testosterone by artificially raising the total count.

For testosterone replacement therapy evaluation, clinicians often look at both total and bioavailable testosterone. If total testosterone appears normal but a patient has symptoms of deficiency, a bioavailable testosterone test can reveal whether SHBG binding is the actual problem. This distinction helps providers at endocrinology and hormone clinics distinguish true hypogonadism from cases where testosterone is present but bound and unavailable. Some labs report bioavailable testosterone directly; others calculate it from free and albumin-bound percentages.