Free Testosterone Test

Testosterone is often thought of as the male sex hormone, but it plays a crucial role in both men and women. It supports energy levels, muscle growth and maintenance, motivation, mood, brain function, bone strength, and sexual health across all genders. In the bloodstream, testosterone travels in two primary forms: bound and free. The majority is bound to proteins like sex hormone–binding globulin (SHBG) and albumin, which helps regulate how much is available to your tissues. Only a small fraction, roughly 1 to 3 percent, circulates freely in the blood, and this free testosterone is the form that’s biologically active. It’s the portion that enters cells and turns on testosterone-responsive genes and processes.

The Free Testosterone (Quest Vermeulen) test estimates this active fraction using a well-validated mathematical model originally developed by Dr. Alex Vermeulen. Quest Diagnostics applies a refined version of this formula to calculate free testosterone from your total testosterone, SHBG, and, when available, albumin. This calculated value gives a clearer picture of how much usable testosterone is actually available to your cells, especially in situations where total testosterone may be misleading.

This method is significantly more reliable than older analog-based assays, which are no longer recommended by most experts. It’s especially useful in cases where SHBG levels are abnormal, which can happen with aging, insulin resistance, obesity, thyroid dysfunction, or liver disease. For example, someone may have normal total testosterone but still experience symptoms of low testosterone (like fatigue, low libido, or muscle loss) if SHBG is high and free testosterone is low. Calculating free testosterone helps uncover these hidden imbalances and is a valuable tool in both diagnosing hormone deficiency and guiding treatment decisions in men and women alike.