Free testosterone
Also called Free T, unbound testosterone, bioavailable testosterone
Free testosterone is the portion of testosterone in your blood that is not attached to any protein, which makes it the portion your tissues can use directly. It is only a small share of the total; most testosterone is bound to SHBG (sex hormone-binding globulin) or, more loosely, to albumin.
Total testosterone counts everything, bound and free. Free testosterone counts only the unbound part. SHBG is the link between them: when SHBG is high, more testosterone is held back and the free share is smaller; when SHBG is low, the free share is larger. A related measure, bioavailable testosterone, adds the loosely albumin-bound portion to the free portion. Free testosterone can be measured directly or estimated from total testosterone, SHBG and albumin, and the Endocrine Society calls for accurate assays for both total and free testosterone.
Why it matters after 45
SHBG tends to shift in midlife, and the direction depends on the person: carrying extra weight, type 2 diabetes and insulin resistance are linked with lower SHBG, while liver disease, an overactive thyroid and estrogen-containing medicines are linked with higher SHBG. When total testosterone sits near the edge of a range, or symptoms and the total result do not agree, free testosterone is often the measure that clarifies the picture.
Where it comes up
- Getting the testosterone test rightwhen free testosterone is added and how it is measured
- Testosterone levels by agehow results, including free testosterone, are interpreted
- When your level is borderlinethe situation where the free fraction is most often consulted
- Testosterone in womenwhy free testosterone matters in women too
Related terms
References
- MedlinePlus. Testosterone Levels Test. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/testosterone-levels-test/
- MedlinePlus. SHBG Blood Test. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/shbg-blood-test/
- Endocrine Society. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline, 2018. https://www.endocrine.org/clinical-practice-guidelines/testosterone-therapy
Definitions here are educational and are not medical advice. They explain what a term means; the pages linked above cover what it means for you, and a clinician reviews your own results. Editorial & medical review policy · All glossary terms