Free Thyroxine (FT4)
- Sample Type
- Standard
- Fasting
- No
- Unit
- ng/dL
Free Thyroxine (FT4) measures the unbound, metabolically active portion of the T4 hormone produced by the thyroid gland. Unlike Total T4, FT4 is not affected by changes in blood protein levels (such as during pregnancy or liver disease). In a TSH and FT4 Panel, it is used to assess thyroid function directly. It helps distinguish between primary thyroid disorders (where TSH and FT4 move in opposite directions) and central/secondary disorders (where they may move in the same direction).
Check Your Result
Check Your Result
Enter your lab result to see where you stand compared to the standard reference range.
Why Context Matters
Recent use of heparin, high doses of biotin (Vitamin B7) supplements, or severe non-thyroidal illness ('euthyroid sick syndrome') can interfere with assay results, leading to misleadingly high or low FT4 readings despite normal thyroid function.
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
- T4 Test - Testing.comTesting.com
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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