The purpose of the present study is to evaluate the diagnostic value of the ETR test as compared to other thyroid function tests in normal persons, patients with thyroid disorders and patients with alterations of thyroxine-binding proteins. The ETR values were obtained from 35 cases as normal control, 63 hyperthyroid patients, 56 euthyroid patients, 23 hypothyroid patients, 10 pregnant women, 5 women taking oral contraceptive medication, 8 liver cirrhosis patients and 4 nephrotic syndrome patients. The results obtained were as follows. 1. The mean value of ETR obtained from the normal controls was $0.99{pm}0.06$. 2. The mean ETR values of various thyroid states were $1.25{pm}0.16$ in hyperthyroidism, $0.99{pm}0.08$ in euthyroidism and $0.82{pm}0.05$ in hypothyroidism and significant difference was found between these groups. 3. Seven out of 63 hyperthyroid patients(11.1%) and 2 out of 23 hypothyroid patients(8.7%) had ETR values within normal range and among the 56 euthyroid patients 6(10.7%) had ETR values outside normal range, so the diagnostic compatibility of ETR was 89.4% in thyroid diseases. 4. Even though the ETR value was well correlated with $^{131}I$-thyroid uptake rate, serum $T_3$ resin uptake rate and serum $T_4$, a high positive correlation was found (r=0.79) between ETR and $T_7$. 5. The mean ETR values from patients with alteration in TBG binding capacity were $0.99{pm}0.05$ in pregnant women, $0.98{pm}0.04$ in women with oral contraceptive medication, $1.04{pm}0.09$ in liver cirrhosis patients and $0.94{pm}0.02$ in nephrotic syndrome patients and most of them (85.2%) had ETR values within normal range. Our results, therefore, suggests that the ETR estimation does offer the simplest and most reliable single procedure for the screening and diagnosis of various thyroid diseases as a indirect indicator of serum-free thyroxine concentration without essential influence of changes in the thyroxine-binding proteins in serum.