Slight emaciation after iodine 131 treatment may be normal, and continued observation is recommended; significant emaciation is abnormal, and may be the result of incomplete treatment, malignant tumors and other conditions. Iodine 131 treatment mainly utilizes the thyroid gland’s ability to highly ingest and concentrate iodine, as well as the ability of iodine 131 to release beta rays, which destroys the follicular cells of the thyroid gland and achieves the therapeutic purpose. Treatment with iodine 131 usually results in an adverse reaction of decreased thyroid hormone levels, which can easily lead to symptoms of hypothyroidism. Patients with hypothyroidism are usually less likely to lose weight and more likely to gain weight. Therefore, if there is significant weight loss after iodine 131 treatment, it is usually an abnormal phenomenon, which may be due to incomplete treatment, malignant tumors and other conditions. If weight loss occurs after Iodine 131 treatment, it is recommended to go to the hospital in time to clarify the cause.