Among the serious complications associated with radioiodine therapy (RAI) for thyroid cancer, cerebral edema is uncommon and has been reported previously in cases of swelling of brain metastases. This case is of a patient with papillary thyroid carcinoma who complained of nausea and vomiting after RAI and was then found unconscious the next day. Laboratory results showed electrolyte imbalances including hyponatremia and cranial imaging only revealed cerebral edema, and she regained consciousness after sodium correction. The etiology of the cerebral edema here is likely multifactorial - due to hyponatremia from hypothyroidism, aggravated by vomiting, copious water intake and a low sodium diet.