Free Thyroxine (FT4)
- Sample Type
- Standard
- Fasting
- No
- Unit
- ng/dL
Free Thyroxine (FT4) is the metabolically active, unbound fraction of thyroxine. Unlike total T4, FT4 is not affected by changes in the concentration of thyroid-binding proteins (like TBG), which can fluctuate due to pregnancy, estrogen therapy, or liver disease. In a panel including TSH, FT4 is critical for distinguishing between primary thyroid disorders (where TSH and FT4 move in opposite directions) and central/secondary thyroid disorders (where TSH and FT4 may move in the same direction).
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Enter your lab result to see where you stand compared to the standard reference range.
Why Context Matters
Biotin supplements can interfere with many FT4 immunoassays, causing falsely elevated results. Severe non-thyroidal illness ('euthyroid sick syndrome') can also cause abnormal FT4 levels that do not accurately reflect true thyroid status.
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 Thyroxine (FT4):
Official Sources
- ATA Guidelines & Statementsthyroid.org
- May 2025 | American Thyroid Associationthyroid.org
Research & Guidelines
- PubMed: Free Thyroxine (FT4) Clinical GuidelinesNCBI / PubMed
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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