Hyperthyroidism in patients with hypothyroidism may be caused by overdose of thyroxine drugs or transient hyperthyroidism during the course of autoimmune thyroiditis. 1. Overdose of thyroxine drugs: due to the lack of thyroxine in the body of patients with hypothyroidism, it is often necessary to supplement the exogenous thyroxine drugs such as levothyroxine sodium tablets to improve the symptoms of hypothyroidism, the patient needs to take the drug according to the doctor’s orders on time and quantity. If the patient takes more than the therapeutic dose of thyroxine medication within a certain period of time, there may be an overdose of thyroxine in the body, thus manifesting the symptoms of hyperthyroidism. 2. In autoimmune thyroiditis such as Hashimoto’s thyroiditis, there are antibodies against the thyroid gland, which can attack the thyroid follicles and cause a large amount of thyroxine to be released transiently, and then under the action of a large amount of thyroxine in the body, the patient can experience a series of hyperthyroidism symptoms. When hypothyroidism patients suddenly appear hyperthyroidism symptoms should check whether they are taking the correct dose of medication, seek professional medical advice, in case of serious symptoms need to consult a doctor in a timely manner.