Free thyroxine index or effective thyroxine ratio?

The ability of the free thyroxine index (FTI) and the effective thyroxine ratio (ETR) to distinguish abnormal from normal thyroid function is compared in a series of 422 patients; they were equally successful in detecting hyperthyroidism. In hypothyroidism, however, the ETR values of 22% were within 95% 'normal' limits compared with 7,4% of the FTI values. Other authors have shown a similar poorer sensitivity of the ETR compared with the FTI.