Hormone receptor abnormality is a syndrome of an abnormal mechanism caused by defective receptor function in hormone action. Resistance to thyroid hormone is a syndrome in which the responsiveness of the target organ to thyroid hormone is reduced. Resistance to thyroid hormone exhibits unsuppressed thyrotropin(TSH) despite elevated free thyroxin (FT4) and free 3,5,3'-triiodothyronine (FT3), termed the syndrome of the inappropriate secretion of TSH (SITSH). Resistance to thyroid hormone is mainly caused by a mutation in the thyroid hormone receptor beta (TRbeta) gene. Genetic analysis of the TRbeta gene is important to diagnose resistance to thyroid hormone. TSH receptor (TSHR) abnormality is classified as a gain-of-function mutation and loss-of-function mutation. Loss-of-function mutations in the TSHR gene occur as TSH resistance, which is found to have euthyroid hyperthyrotropinemia or hypothyroidism because of the reduced responsiveness of the receptor to TSH. R450H mutation in the TSHR gene is occasionally observed in Japanese patients with TSH resistance. In Japan, it is suggested that analysis of the R450H mutation in the TSHR gene is useful to determine the cause of hyperthyrotropinemia or hypothyroidism.