Free Thyroxine Index
The free thyroxine index (FTI) is a calculated value that combines total T4 with T3 uptake to estimate your free thyroid hormone. It is an older way to account for protein binding.
Because total T4 alone can be misleading when binding proteins change, the FTI corrects for that by factoring in T3 uptake. This gives a more reliable estimate of active hormone, historically useful in pregnancy or with estrogen use. Direct free T4 testing has largely replaced it, but it may still appear on panels.
A high FTI suggests more available thyroid hormone and can point toward an overactive thyroid. Since it is a calculation, it should be interpreted alongside TSH and clinical context. Your clinician can confirm the significance.
A low FTI suggests less available thyroid hormone and can point toward an underactive thyroid, though results should be read with TSH and your overall picture.
Target ranges depend on the specific lab's method, and FTI is best interpreted together with TSH. Your clinician can explain where your value falls and whether direct free T4 testing is preferable.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.