The Free Thyroxine Index (FTI) was developed to correct for variations in thyroid hormone-binding proteins that can distort total T4 measurements. It combines total T4 with T3 uptake (a measure of available binding sites) to estimate free T4. Modern direct free T4 assays have largely replaced FTI, but it remains useful in selected cases.
This test is used to evaluate thyroid function when binding protein abnormalities are suspected, such as in pregnancy, severe illness, or on medications affecting thyroid hormone binding. Sample type is serum.
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