What is free testosterone?

Free testosterone refers to testosterone molecules that circulate in the bloodstream without being attached to carrier proteins. Only this unbound fraction can pass through cell membranes and bind to androgen receptors, making it the form that actually produces the effects associated with testosterone: muscle growth, bone density, sexual function, mood, and energy levels.

Total testosterone measures all testosterone in the blood, including the free portion plus testosterone bound to sex hormone-binding globulin (SHBG) and albumin. In many cases, total testosterone appears normal while free testosterone is low, particularly in older men or those with high SHBG levels. This distinction matters because a patient can have adequate total testosterone but experience symptoms of deficiency if most of it is bound and unavailable.

Physicians typically order a free testosterone test when symptoms suggest low testosterone (low libido, fatigue, muscle loss, mood changes) but total testosterone results are borderline or inconsistent with symptoms. Direct measurement of free testosterone provides a clearer picture of hormone availability. Some labs report free testosterone as a percentage (free testosterone index), while others measure it directly in picograms per milliliter.

When evaluating testosterone replacement, endocrinology and hormone specialists often check both total and free testosterone to establish a baseline and determine whether treatment is necessary. Monitoring free testosterone during therapy helps clinicians adjust dosing to bring levels into the therapeutic range without oversupplying.