The active form of thyroid hormone — measures what's biologically available.
What is it
Free T3 (triiodothyronine) is the biologically active thyroid hormone unbound to carrier proteins. It is the form that interacts with thyroid hormone receptors in tissue.
Why it's measured
Free T3 has been studied in research on cellular thyroid activity and is often considered alongside Free T4 in comprehensive thyroid evaluation.
Reference range
Standard laboratory reference
2.3 – 4.2 pg/mL
Reference ranges vary by laboratory. Your individual reference range will appear on your test report and should be interpreted by your physician in the context of your overall health profile.
What results may indicate
Elevated
Has been associated with hyperthyroidism in published research.
Lower
Has been associated with hypothyroidism and certain non-thyroidal illnesses in research literature.
These associations are general. They are not a personal diagnosis or prediction. Discuss your individual results with your physician in the context of your full health profile.
When it's measured
Used in comprehensive thyroid evaluation, especially when TSH alone is insufficient.
Related markers
Measured in
A complete look at your heart, hormones, metabolism, inflammation, and longevity systems — drawn at Quest or LabCorp, reviewed by a U.S.-licensed physician.
Explore the Apex Panel →This information is for general educational purposes only and is not medical advice. Lab results alone are not intended to diagnose, treat, or cure any condition and do not replace the advice of a healthcare provider. OPTML does not offer medical advice, a diagnosis, medical treatment, or any form of medical opinion.