Indirect estimation of the free T4 in the blood is known as which index?

• A. FTI
• B. TT4
• C. T3U
• D. THBR

Answer: (A)

The concept here is estimating the active hormone level without directly measuring it. Free T4 is the portion of thyroxine not bound to transport proteins and is what tissues actually use. Because most T4 is bound to thyroxine-binding globulin, direct free T4 assays can be tricky and influenced by binding protein levels. The Free Thyroxine Index uses a simple combination of two measurements—total T4 and T3 uptake (a surrogate for the binding capacity of TBG)—to approximate the free fraction of T4. By multiplying TT4 by the T3 uptake value, clinicians get an indirect estimate of free T4 that reflects changes in binding proteins, without needing a direct free T4 measurement. In practice, this index has been used historically for assessing thyroid status when binding proteins vary. The other options measure different things: total T4 is the overall amount of hormone regardless of binding, T3 uptake (T3U) is the binding surrogate by itself and isn’t the specific index for estimating free T4, and THBR is a separate binding ratio indicator.