The radioimmunoassay of human thyrtropin was performed in various thyroid diseases, using the antin-h-TSH antibody(Daiichi, Japan) and purified h-TSH(Biodata, Italy), 125I labelling of h-TSH was performed using a small amount(l.8 μg) of chloramin-T as an oxidant at room temperature. This new method facilitated uniform labelling and reduced to damage of h-TSH by chloramin- T, and satisfactory radioimmunoasasy results were obtained with 125I-TSH prepared by the new method until at least 7 weeks after preparation. The assay sensitivity was 1.0 μu/ml. The coefficient of variances in within assay and interrun assay were all less than 10% among the range of 1.25 μu/ml and 160 μu/ml. The serum TSH values by this new method were colsely related to those by commercial kit(r=0.996).