RESUMEN
Serum total thyroid hormones (TT4 and TT3) have been influenced by alterations in the serum concentration of thyroxine-binding globulin (TTG), and it is necessary to determine the concentrations of free thyroid hormones (FT4 and FT3) which are the biologically active component. However, free T3 index (FT3I) or free thyroxine index (FT4I) measurements are generally accepted clinically so the usefulness and applicability of our FT3I by an in-house method euthyroids, thyroid dysfunction and patients with altered TBG were evaluated. The results indicated that the mean value of serum FT3I in 331 euthyroid subjects was 1.61 + 0.33% (+ SD) with the range of 0.90 – 2.60%. AII the FT3I values in 514 normal pregnancies and 132 chronic renal failure patients were within the normal levels which showed no overlaping results with those of 37 hypothyroids 132 hyperthyroids and 65 pregnant women with hyperthyroidism. Our FT3I gave a more precise index of thyroid status since the data correlated well with clinical observation and the usually routine test of thyroid function. The FT3I has advantages when TBG level are abnormally high or low. Total .T3 (TT3) may be elevated without an elevated TT4 in T3-toxicosis, and thus the FT3I may prevent a misdiagnosis of T3-toxicosis as demonstrated in 61 patients with T3-toxicosis. It is concluded that our FT3I would be very useful in patients with altered TBG or binding capacity and suspected T3-toxicosis.