The radioimmunoassay of human thyrotropin was performed in various thyroid states, utilizing the anti-h-T.S.H. antibody and purified human thyrotropin supplied from National Institute of Arthritis and Metabolic Diseases, Bethesda, Ma., U.S.A., and human thyrotropin standard-A obtained from National Institute for Biologic Standards, Mill Hill, London, England. 131I labelled h- TSH was prepared after the Chloramine-T method of Greenwood et al. This double antibody system had a assay sensitivity of about l. 0 μU/ml of plasma HTS-A and could detect the plasma h-TSH level in the euthyroid patients. Plasma h-TSH level of the normal 26 Korean was l. 1±0. 83 μU/ml, and that of the 8 hypothyroidisms were 8. 3 to 67. μU/ml. In hyperthyroidisms, no cases showed the plasma h-TSH levels over l. 0μU/ ml. Between the hypothyroidism and euthyroidsm, no overlap is noticed on plasma h-TSH levels. A case of transient hypothyroid state identified by determination of plasma h-TSH level is presented. These results revealed that the radioimmunoassay of h-TSH in plasma could be a sensitive method to diagnose the hypothyroidsm, if not caused by a pituitary disease.