Normalized T4 (T4N), total T3, T3 resin sponge uptake (T3U) and the response of TSH to TRH were determined in 264 patients sent for diagnosis of thyroid function. Many of these patients were diagnostic problem cases. Those with disease or medication known to invalidate the TRH test, but not those with abnormal thyroid hormone binding capacity in serum, were excluded. A free T3 index (FT3 index) was calculated as the product of total T3 and T3U. The FT3 index tended to decline with increasing age, the mean value being significantly higher in the youngest patients (12-20-years-old) than in the older age groups. In the diagnosis of hyperthyroidism, the FT3 index was superior not only to T4N but also to total T3, particularly in the presence of elevated hormone binding capacity (relatively low T3U). It is anticipated that the FT3 index will be most useful as a routine test for hyperthyroidism, especially in several (though not all) instances where the TRH test is invalidated.