What is free testosterone?

Free testosterone represents the portion of testosterone in your blood that is not attached to carrier proteins. Most testosterone (95-98 percent) binds to sex hormone-binding globulin (SHBG) or albumin, making it inactive or slowly available to tissues. Only the unbound fraction can enter cells and activate androgen receptors to produce the effects men seek from testosterone therapy: increased muscle mass, energy, libido, and strength.

Total testosterone measures all forms together. A man might show a normal total testosterone level while having insufficient free testosterone available to his tissues, or conversely, have a low total but adequate free levels if his SHBG is low. This distinction matters because free testosterone more accurately reflects what your body can actually use.

Testosterone replacement therapy clinics in Florida test free testosterone because it predicts clinical outcomes better than total testosterone alone. A patient's symptoms, side effects, and therapy adjustment hinge on this active fraction. Some labs report free testosterone by direct measurement (dialysis or ultrafiltration), while others calculate it from total testosterone and SHBG levels. Either method gives clinicians the data needed to dose appropriately and monitor whether therapy is producing physiological activity.