What is the reference range for Free Thyroxine Index (FTI)?

• A. 3.0-6.0
• B. 5.4-9.7
• C. 9.0-12.0
• D. 12.0-15.0

Answer: (B)

Free Thyroxine Index is an indirect measure of the amount of free thyroid hormone, estimated by combining a total T4 value with a T3 uptake result. It serves as a surrogate for free T4 when a direct free T4 assay isn’t available, helping to gauge thyroid status in a practical way.

The normal reference range for FTI is about 5.4–9.7. This mid-range interval best captures euthyroid values across many laboratories, so it’s the correct choice. Values falling well below or above this range would raise concern for hypothyroidism or hyperthyroidism, respectively, and would typically prompt correlation with other thyroid tests such as TSH and free T4.