We compared the superiority of the thyroid function studies by RIA and found to have T4, RIA was a single most efficient and reliable method among the other method uses for the thyroid function test. During the 14 months period 1981-1982, 209 patients were showed nomal ranges of T4 uptake study at Nuclear Medicine Department of Korea University Hospitals. We evaluate in those 209 patients with the further studies of T3 uptake and FTI and obtained normal values of T4, RIA in 203 patients. Therefore, T4, RIA study was 97.1% (203/209) accurated retrospectively. Six cases of abnormal T4, RIA value are reported and there are consited of 3 women and 3 men. Among the 6 cases of abnormal T4, RIA, 4 cases of increased FTI and 2 case of decreased FTI were found. One of 4 cases of increased FTI reveals that the patient found to have septicemia and one of 2 cases of decreased FTI reveals that the patient has been on over dose of antithyroid drug. Repeated T4, RIA study were done on rest of 4 cases of abnormal T4, RIA patients and found to have that the results was technical errors. In conclusion, we found that the T4, RIA study was the most reliable, enconomic and efficient test of evaluating the thyroid function.