What is free testosterone?
Free testosterone is the portion of testosterone circulating in the bloodstream unbound to proteins, directly available for cellular action. Clinics measure it to assess the hormonally active fraction independent of total testosterone levels.
In the bloodstream, testosterone exists in three forms: bound tightly to sex hormone-binding globulin (SHBG), bound loosely to albumin, and unbound. Free testosterone is that unbound fraction, typically less than 2 percent of total testosterone. Only free testosterone can cross cell membranes and bind to androgen receptors to produce effects like increased muscle mass, bone density, and sexual function.
Total testosterone measurement alone can miss a key picture of a patient's hormonal status. A man might have a total testosterone level in the normal range but high SHBG levels, meaning little free testosterone reaches tissues. Conversely, low SHBG might mean more bioavailable hormone despite moderate total levels. This distinction matters in TRT clinics across Florida and elsewhere.
When you visit a men's health clinic for testosterone evaluation, providers typically order both total and free testosterone tests. Free testosterone results, measured in picograms per milliliter (pg/mL), help clinicians determine whether symptoms like fatigue, low libido, or reduced muscle strength reflect true hormone deficiency or simply poor bioavailability. Some clinics also assess SHBG to understand what's driving the free fraction up or down, informing treatment decisions and dosing.