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). $^{125}I$ labelling of h-TSH was performed using a small amount $(1.8{mu}g)$ 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 $^{125}I-TSH$ prepared by the new method until at least 7 weeks after preparation, The assay sensitivity was $1.0{mu}u/ml$. The coefficient of variances in within assay and interrun assay were all less than 10% among the range of $1.25{mu}u/ml$ and $160{mu}u/ml$. The serum TSH values by this new method were colsely related to those by commercial kit (r= 0.996).