Compared to simple anti-thyroid medication, iodine-131 treatment is much more likely to cause hypothyroidism in patients with hyperthyroidism. Hypothyroidism after treatment with iodine-131 therapy is divided into early-onset hypothyroidism and late-onset hypothyroidism. Early onset hypothyroidism is defined as hypothyroidism that occurs within 1 year after iodine-131 treatment, which may be related to factors such as the patient’s thyroid gland being more sensitive to radiation and the high dose of treatment. The incidence of early onset hypothyroidism cannot be accurately predicted by physicians at this time. Lowering the dose of iodine-131 treatment may reduce the incidence of early onset hypothyroidism, but this will reduce the remission rate with a single treatment, and some patients will require repeat treatment (with a subsequent increase in the incidence of hypothyroidism). Some patients with early-onset hypothyroidism (especially young adults) can return to normal thyroid function on their own with adjustments and adjuvant therapy. Late onset hypothyroidism refers to hypothyroidism that occurs after 1 year of iodine-131 treatment. The incidence of late onset hypothyroidism is increasing year by year (by 2% to 3% per year), which may be mainly related to the patient’s abnormal autoimmune function and not to the dose of iodine-131 treatment (in fact, other hyperthyroid treatments can also cause late onset hypothyroidism). Therefore, patients who have received iodine-131 therapy should be reviewed at least once a year to detect changes in thyroid function in a timely manner. Hypothyroidism is easy to diagnose and treat clinically. As mentioned earlier, if a hyperthyroid patient develops fatigue, drowsiness, weight gain, fear of cold, cramps, increased menstrual flow and constipation in young and middle-aged women after iodine-131 treatment (one or more of the above conditions together), it often indicates early-onset hypothyroidism. Hypothyroidism is easily diagnosed by testing serum thyroid hormone levels. And with timely replacement therapy with appropriate amounts of thyroid hormone (eugenol or other thyroid hormone preparations), hypothyroidism can be easily corrected. Only those who go undetected for a long time and whose hypothyroidism gradually worsens will develop serious hypothyroid complications. In conclusion, the key to managing hypothyroidism is early detection and timely thyroid hormone replacement therapy. It is worth mentioning that patients with early onset hypothyroidism who do not recover on their own and patients with late onset hypothyroidism need to be treated with thyroid hormone replacement therapy all the time. Patients should not give up replacement therapy because they mistakenly think that they are completely cured when various test indicators are normal, resulting in severe hypothyroidism. In addition, women who rely on thyroid hormone replacement therapy should also adhere to the medication during pregnancy as prescribed by their doctors and adjust the dosage according to the results of regular checkups.