To clarify clinical evaluation of highly sensitive thyroid stimulating hormone (TSH) immunoradiometric assay (IRMA) using RIABEAD II kit, we measured serum TSH and its changes after TRH test in normal subjects and patients with hyper- or hypothyroidism due to Grave's disease, TSH-producing pituitary adenoma and chronic thyroiditis or Refetoff's syndrome. In 12 normals, basal TSH (0.23-1.85 microIU/ml) increased in 2.29-18.85 microIU/ml 30 min after TRH test. In patients whose serum TSH and thyroid hormone showed less than 0.10 microIU/ml and more than normal level, respectively, no response of TSH to TRH was observed, while in patients whose serum TSH and thyroid hormone showed more than 3.5 microIU/ml and less than normal level, respectively, hyperresponse of TSH to TRH was observed. In patients with hyper- or hypothyroidism, changes in serum TSH after treatment were normalized a few months later after normalizing of thyroid hormone. In patients with TSH-producing pituitary tumor or Refetoff's syndrome, serum TSH increased. These results show that serum TSH using IRMA-(RIABEAD II) kit indicates a precise function of pituitary TSH.