Free Triiodothyronine (FT3)
- Sample Type
- Standard
- Fasting
- No
- Unit
- pg/mL
Free T3 is the unbound and biologically active form of the hormone triiodothyronine. While T4 is produced in greater quantities, T3 is significantly more potent at the cellular level. In a Thyroid Function Test (TFT) panel, FT3 is primarily used to diagnose hyperthyroidism and to assess the severity of thyrotoxicosis. It is particularly useful in 'T3 Toxicosis,' where TSH is low and FT4 is normal, but FT3 is elevated.
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Why Context Matters
FT3 levels can be misleading in 'Non-Thyroidal Illness Syndrome' (Sick Euthyroid Syndrome), where levels drop significantly during acute severe illness or starvation despite no primary thyroid pathology. Second opinions are necessary to differentiate between true hypothyroidism and transient illness-related drops.
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
Research & Guidelines
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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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