Testosterone is often discussed as a single number, but the reality is more nuanced. Most of the testosterone circulating in your blood is bound to proteins, primarily sex hormone binding globulin (SHBG) and albumin. Only a small fraction, roughly 1 to 3 per cent, is unbound and free to enter cells and exert biological effects. This panel measures total testosterone, calculated free testosterone, and SHBG together, giving a fuller picture than total testosterone alone.
Free testosterone matters because it is the portion your body can actually use. Two people with the same total testosterone level can have very different free testosterone levels depending on how much SHBG they produce. SHBG is influenced by age, thyroid function, liver health, body weight, and certain medications. When SHBG is elevated, it binds more testosterone, leaving less available for tissues. This is why symptoms of low testosterone can appear even when total testosterone looks normal on paper.
In men, this panel is commonly used to investigate fatigue, low libido, erectile dysfunction, reduced muscle mass, and poor concentration. In women, it plays a different role: elevated testosterone and free androgen index are part of the hormonal picture in polycystic ovary syndrome (PCOS) and other conditions involving androgen excess.
Timing matters for this test. Testosterone follows a circadian rhythm in men, peaking in the early morning and declining through the day. Collection before 10am is required for results that can be reliably compared against reference ranges. Your practitioner will interpret the full panel, including total testosterone, free testosterone, and SHBG, alongside your symptoms, age, and clinical context.
This test is ideal for men and women experiencing unexplained fatigue, reduced libido, difficulty building muscle, mood changes, or decreased motivation. Particularly valuable for individuals over 30, those on medications that might affect hormone binding, or anyone with normal total testosterone but persistent symptoms. Also useful for monitoring hormone replacement therapy effectiveness and assessing SHBG-related hormone imbalances.