In the Free Thyroxine Index formula, T3U is divided by what value?

• A. Mean normal T3U
• B. Mean normal TBG
• C. Mean normal FT4
• D. Mean normal TT4

Answer: (A)

The question tests how the Free Thyroxine Index (FTI) is calculated. T3 uptake (T3U) is a relative measure that depends on assay conditions, so it is divided by the mean normal T3U to normalize it to a standard baseline. This normalized T3U is then multiplied by the total thyroxine (TT4) to estimate the free thyroxine index. Dividing by mean normal TBG, FT4, or TT4 would not fit the standard calculation, since TT4 is used as the multiplier after normalization, not as the divisor, and TBG or FT4 are not part of this normalization step.