Free Thyroxine (FT4)
- Sample Type
- Standard
- Fasting
- No
- Unit
- ng/dL
Free Thyroxine (FT4) is the active, unbound form of the hormone T4 produced by the thyroid gland. Unlike total T4, FT4 is not affected by changes in levels of binding proteins (like TBG), making it a much more accurate reflection of thyroid function. Within the Free T3, Free T4 panel, it is the primary tool for diagnosing hyperthyroidism and hypothyroidism and monitoring treatment with synthetic thyroid hormones.
Check Your Result
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 ('Sick Euthyroid Syndrome') can cause abnormal FT4 levels despite a healthy thyroid. Additionally, high doses of Biotin (Vitamin B7) supplements can cause analytically false results in many lab assays, often mimicking hyperthyroidism (high FT4, low TSH).
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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