The main cause of death in patients with hyperthyroid heart disease is heart failure. If patients with hyperthyroid heart disease are not treated with regular antithyroid medications or medications to improve cardiac function, their survival is expected to be about 1-2 years. On the other hand, if a patient with hyperthyroidism is treated with effective antithyroid medication or radioactive iodine-131 therapy to keep thyroid hormone levels within normal limits, and oral medication to improve cardiac function, the patient’s survival is expected to be about 5-10 years. These are relative figures, but if a patient with hyperthyroidism suffers an exacerbation of heart failure or acute left heart failure due to infection or other factors, the patient may die within a very short period of 1-2 months.