What is free testosterone?
Free testosterone is the small portion of testosterone in the blood that is not bound to proteins and is therefore available for cells to use.
Most testosterone in the bloodstream is bound to sex hormone-binding globulin (SHBG) or albumin, making it unavailable for cells to use. Free testosterone, typically 1-3 percent of total testosterone, circulates unattached and can enter cells to produce biological effects like increased muscle mass, bone density, energy, and sexual function.
TRT clinics test free testosterone because total testosterone levels alone do not always reflect how much hormone is actually active in the body. A patient may have normal total testosterone but low free testosterone if binding proteins are elevated, or vice versa. Free testosterone correlates more directly with symptoms such as fatigue, low libido, muscle weakness, and mood changes. For this reason, clinics often order both total and free testosterone tests to make accurate treatment decisions.
Reference ranges for free testosterone in adult men vary by assay method but typically fall between 8.7 and 25 pg/mL (picograms per milliliter) or 0.3 to 0.9 ng/dL (nanograms per deciliter), depending on the laboratory. Some clinics may use equilibrium dialysis or liquid chromatography-mass spectrometry as more precise measurement methods. Low free testosterone, even with normal total levels, may indicate a need for hormone therapy evaluation and monitoring throughout treatment.