Value of the free triiodothyronine index in the diagnosis of hyperthyroidism

Abstract

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.

Letters to the Editors

“…Neither was total T3 superior to total T4 in the diagnosis of hyperthyroidism. At first sight this is at variance with many reports published since the classical paper of Clark & Horn (1965) on the FT4 index and articles like that of Marsden & McKerron (1975) on T3 and of Stepanas et al (1977) and myself (Wiener, 1980) on the FT3 index.…”

“…As for the inclusion of diagnostic problem cases, the validity of the final diagnosis is of crucial importance if the diagnostic value of tests is to be compared. With the TRH test as final criterion, after exclusion of patients with disease or medication known to invalidate this test, but not those with abnormal TBG capacity, I found the FT3 index to be clearly superior not only to normalized T4 (a measure of free T4) but also to total T3 in the differentiation of eu-and hyperthyroidism (Wiener, 1980).…”