What is bioavailable testosterone?

Bioavailable testosterone represents the sum of free testosterone and albumin-bound testosterone. This measurement matters because only unbound or loosely bound testosterone can enter cells and produce biological effects. The remainder of total testosterone binds tightly to sex hormone-binding globulin (SHBG), making it unavailable for cellular use.

Most laboratories calculate bioavailable testosterone using an equation based on total testosterone, free testosterone, and albumin levels, rather than measuring it directly. The formula typically estimates the portion of testosterone that dissociates from albumin at tissue pH, which is clinically relevant for symptom assessment.

In testosterone replacement therapy, bioavailable testosterone often correlates better with clinical symptoms than total testosterone alone, especially in patients with unusual SHBG levels. Conditions like liver disease, obesity, or age-related changes can alter SHBG levels and shift the ratio between bound and bioavailable hormone. Some providers order this value alongside total and free testosterone to clarify whether symptoms align with hormonal status or other causes. When total testosterone appears low but free testosterone seems normal, or vice versa, bioavailable testosterone helps contextualize the results. Diagnostic labs in Florida typically report all three values (total, free, and bioavailable) to give clinicians a complete picture of available hormone.