Free Triiodothyronine (FT3)
- Sample Type
- Standard
- Fasting
- No
- Unit
- pg/mL
Free T3 measures the unbound, metabolically active form of triiodothyronine. While T4 is produced in higher quantities, T3 is the more potent thyroid hormone. In the Thyroid Function Panel, FT3 is essential for diagnosing 'T3 toxicosis' (where T3 is elevated but T4 is normal) and for monitoring the severity of hyperthyroidism. It provides a clearer picture of thyroid status than Total T3 because it is not affected by changes in thyroid-binding globulin (TBG) levels caused by pregnancy or medications.
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Enter your lab result to see where you stand compared to the standard reference range.
Why Context Matters
FT3 levels can be misleadingly low in 'Euthyroid Sick Syndrome' during acute or chronic non-thyroidal illness. Additionally, high doses of Biotin (Vitamin B7) supplements can interfere with the immunoassay, leading to falsely elevated FT3 results, which could lead to a misdiagnosis of hyperthyroidism.
Lab ranges are statistical averages, not biological laws. "Normal" for a 20-year-old male isn't normal for a 60-year-old female.
Clinical References
Source-of-truth databases and clinical guidelines for Free Triiodothyronine (FT3):
Official Sources
- ATA Guidelines & Statementsthyroid.org
Research & Guidelines
Related Indicators
Medically Reviewed by Dr. Binoy Babu, MBBS
Board Certified Doctor • 10+ Years Clinical Experience
Dr. Babu is a practicing physician dedicated to empowering patients with clear, actionable medical information. He founded 2opi to bridge the gap between complex lab reports and patient understanding, ensuring everyone has access to a reliable second opinion.
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