In the blood, the majority of testosterone (approximately 98%) is bound to proteins—mainly sex hormone-binding globulin (SHBG) and albumin. Only the remaining 1% to 2% is completely unbound, known as free testosterone.
Total testosterone provides an overall measure of hormone production. However, conditions such as obesity, thyroid disease, diabetes, and aging can significantly alter SHBG levels, making total testosterone appear normal even when active hormone availability is low, or vice versa.
Evaluating both total and free (or bioavailable) testosterone gives the physician a complete picture of whether your body has sufficient bioavailable hormone to support normal physiological functions.
Risks and limitations
- Free testosterone assays must be performed using reliable testing methods (such as equilibrium dialysis or calculated free testosterone) to ensure clinical accuracy.
- A low free testosterone number must always be interpreted in the context of clinical signs and symptoms, not in isolation.