Free thyroxine (FT4) values determined by direct radio-immunoassay and the free thyroxine index (FTI) determined by indirect calculation were compared in 431 consecutive sera from patients with suspected thyroid dysfunction . Sensitivity and specificity were similar for both tests in the thyrotoxic, hypothyroid and euthyroid ranges with appropriate thyroid-binding globulin (TBG) concentrations. However, FT4 was superior to FTI in the euthyroid group with high concentrations of TBG, FT4 is therefore a useful alternative to FTI as a screening test for thyroid dysfunction, and is likely to be a better discriminator of thyroid status in patients with high TBG levels.