The Free Thyroxine Index (FTI or T7) is obtained by multiplying the total T with T uptake. FTI is considered to be a more reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding. This test is rarely used now that reliable free thyroxine and free triiodothyronine assays are routinely available.
FTI is elevated in hyperthyroidism and decreased in hypothyroidism.