Free Thyroxine (FT4)
- Sample Type
- Standard
- Fasting
- No
- Unit
- ng/dL
Free Thyroxine (FT4) measures the unbound portion of thyroxine, which is the biologically active form of the hormone available to tissues. Unlike Total T4, FT4 is not influenced by changes in the concentration of thyroid-binding globulin (TBG). This makes it a superior diagnostic tool for assessing thyroid status, especially in patients who are pregnant, taking oral contraceptives, or have liver/kidney disease. Within a Thyroid Function Test panel, FT4 is used alongside TSH to diagnose hyperthyroidism (high FT4, low TSH), primary hypothyroidism (low FT4, high TSH), or central hypothyroidism (low FT4, low/normal TSH).
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Check Your Result
Enter your lab result to see where you stand compared to the standard reference range.
Why Context Matters
Severe non-thyroidal illness ('Euthyroid Sick Syndrome') or medications like amiodarone, heparin, and high-dose aspirin can transiently alter FT4 levels without primary thyroid pathology. Biotin supplements can also interfere with many FT4 immunoassays.
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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