The main manifestation is an acute exacerbation or worsening of symptoms associated with hyperthyroidism. Although the onset or complication of hyperthyroidism after 131I therapy is rare, clinical vigilance is required. To prevent the occurrence of hyperthyroidism crisis, patients with severe hyperthyroidism may be considered to be treated with anti-thyroid drugs and symptomatic treatment before 131I treatment. If the patient develops a condition similar to hyperthyroid crisis after 131I treatment, he/she should be treated promptly at the nearest major hospital internal medicine department or emergency room. The following conditions should be considered as critical symptoms in patients with hyperthyroidism: high or excessive fever, profuse sweating, tachycardia (>140 beats/min), irritability and anxiety or even delirium, severe vomiting or diarrhea, or heart failure, shock and coma. The diagnosis of hyperthyroidism crisis is mainly based on the comprehensive judgment of clinical manifestations. The mortality rate of hyperthyroidism crisis can be more than 20%.