Free Thyroxine (FT4)
- Sample Type
- Standard
- Fasting
- No
- Unit
- ng/dL
Free Thyroxine (FT4) measures the concentration of thyroxine that is not bound to carrier proteins (like Thyroid-Binding Globulin). It is the biologically active form of T4 available to body tissues. Within a Thyroid Function Panel, FT4 is superior to Total T4 because it is unaffected by changes in protein levels (e.g., during pregnancy or use of oral contraceptives). It is essential for diagnosing hypothyroidism and hyperthyroidism and for monitoring patients on thyroid replacement therapy.
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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
Certain medications (like heparin, amiodarone, or high-dose biotin) can interfere with laboratory assays. Severe non-thyroidal illness ('Euthyroid Sick Syndrome') can also cause abnormal results despite a healthy thyroid.
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
- Hypothyroidism - StatPearls - NCBI Bookshelfncbi.nlm.nih.gov
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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