The Free Thyroxine Index (FTI) is the calculated product of T-3 resin uptake and serum total thyroxine (T-4) and is the best initial screening test for hyperthyroidism. It permits correction of misleading results in T-3 and T-4 determinations caused by conditions that alter the thyroxine-binding protein concentration (e.g., pregnancy, estrogens, birth control pills). The American Thyroid Association now recommends the term thyroid hormone-binding ratios (THBR).

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