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.