Hyperthyroidism may develop into hypothyroidism after iodine 131 treatment. The mechanism by which hypothyroidism occurs in hyperthyroidism treated with iodine 131 is unclear, and hypothyroidism after iodine 131 treatment can be categorized as early-onset hypothyroidism and late-onset hypothyroidism. Early-onset hypothyroidism refers to hypothyroidism 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 excessive dose of iodine 131 treatment. Some patients with early-onset hypothyroidism (especially young patients) may recover thyroid function on their own. Late-onset hypothyroidism refers to hypothyroidism that occurs 1 year after iodine 131 treatment. Hypothyroidism after iodine 131 treatment is easy to diagnose and treat clinically, and only those who remain untreated for a long period of time and whose hypothyroidism gradually worsens will develop serious complications of hypothyroidism. The key to dealing with hypothyroidism is early detection and timely thyroid hormone replacement therapy. If you feel unwell after iodine 131 treatment for hyperthyroidism, it is recommended that you consult a doctor promptly.