Which expression represents the FTI formula?

Which expression represents the FTI formula?

• A. TT4 x (T3U/ Mean normal T3U)
• B. TT4 x THBR
• C. TT4 ÷ (T3U/ Mean normal T3U)
• D. TT4 + T3U

Answer: (A)

Free thyroxine index (FTI) estimates the amount of free T4 by combining the total T4 with a measure of thyroid hormone binding capacity, then normalizing to a reference binding value. The T3 uptake test serves as a proxy for binding capacity—when binding proteins are high, T3 uptake tends to be lower, and when binding is low, T3 uptake tends to be higher. By multiplying TT4 by the T3 uptake and then dividing by the mean normal T3 uptake, you adjust for individual differences in binding protein levels and obtain an index that tracks free T4 levels. This is why the correct expression is TT4 × (T3U / Mean normal T3U). Using TT4 with no normalization, or using inappropriate operations like addition, would not correctly reflect the free portion of T4 across different binding conditions.

Free thyroxine index (FTI) estimates the amount of free T4 by combining the total T4 with a measure of thyroid hormone binding capacity, then normalizing to a reference binding value. The T3 uptake test serves as a proxy for binding capacity—when binding proteins are high, T3 uptake tends to be lower, and when binding is low, T3 uptake tends to be higher. By multiplying TT4 by the T3 uptake and then dividing by the mean normal T3 uptake, you adjust for individual differences in binding protein levels and obtain an index that tracks free T4 levels. This is why the correct expression is TT4 × (T3U / Mean normal T3U). Using TT4 with no normalization, or using inappropriate operations like addition, would not correctly reflect the free portion of T4 across different binding conditions.