The FTI is calculated from the results of a total T4 estimation and a thyroid hormone binding test. Depending upon the method used for the thyroid hormone binding test, the calculation is as shown in Fig. 15.13. The FTI gives a value for T4 which is corrected for alterations in TBG capacity and is of particular value in subjects taking oral contraceptive therapy, in pregnancy, and in hypoproteinaemic states. The index has been widely used for these reasons, but it involves the use of two tests and therefore may be superseded by the direct ΓÇÿfreeΓÇÖ thyroid hormone assays, which effectively measure the same entity.
