Evaluation of the Free Triiodothyronine Index (FT3I) in Various Thyroid Conditions

Serum total thyroid hormones (TT 4 and TT 3 ) 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 (FT 4 and FT 3 ) which are the biologically active component. However, free T 3 index (FT 3 I) or free thyroxine index (FT 4 I) measurements are generally accepted clinically so the usefulness and applicability of our FT 3 I by an in-house method euthyroids, thyroid dysfunction and patients with altered TBG were evaluated. The results indicated that the mean value of serum FT 3 I in 331 euthyroid subjects was 1.61 ± 0.33% (± SD) with the range of 0.90 - 2.60%. AII the FT 3 I 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 FT 3 I 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 FT 3 I has advantages when TBG level are abnormally high or low. Total .T 3 (TT 3 ) may be elevated without an elevated TT 4 in T 3 -toxicosis, and thus the FT 3 I may prevent a misdiagnosis of T 3 -toxicosis as demonstrated in 61 patients with T 3 -toxicosis. It is concluded that our FT 3 I would be very useful in patients with altered TBG or binding capacity and suspected T 3 -toxicosis.