ABSTRACT
Serum total thyroxine (TT4) measurement by radioimmunoassy (RIA) has been the most requested test of thyroid function, but high or low values may occur in euthyroid persons as a consequence of alterations in the concentrations or affinities of the serum binding proteins. The free thyroxine index (FT4I) is belive to be an ‘excellent approximation’ of the true concentration of free thyroxine (FT4), which has been shown to define thyroid status in the presence protein abnormality. Our FT4I has been developed by in-house method since 1981. Its value is derived from the ratio of TT4 to T3-uptake that corrects the TT4 for variations in thyroxine-binding proteins. The aim of this study is to compare and evaluate the values of our FT4I and of the Amerlex-M FT4 RIA kit, which is commercially available and is used as a reference method from 662 subjects divided into 5 categories; euthyroid as control, pregnant woman, chronic renal failure (CRF), hyperthyroid and hypothyroid. The results revealed that the values (+SD) of TT and T-uptake in pregnant women and in CRF are deviated from the control, but all the values of FT4I and FT4 in these subjects are within the non-pregnant euthyroid ranges. Our results also indicated that the mean values of FT4I and FT4 in euthyroid, hyperthyroid and hypothyroid differed from one another significantly (p<0.001). Moreover, significant correlations (p<0.01) of the both techniques are obtained in all categories of the subjects, utilizing Spearman’s rank correlation coefficient (rs). Therefore, our FT4I is a reliable substitute for the Amerlex FT4 assay kit in thyroid function test.