The etiology of psychotic symptoms that emerge following surgery needs to be sought to allow physicians to provide effective treatment. We present the case of a patient who developed hyperthyroidism shortly after excision of a mandible bone tumor and discuss its clinical features, course, and management. A 48-year-old female without previous thyroid disease accidentally found a tumor over her left mandible bone and underwent excision of the tumor. Soon after surgery, she suffered from anxiety, mood swings, insomnia, and even auditory hallucinations. Through careful differential diagnosis and a series of examinations, she was shown to be in a hyperthyroid state. Her condition improved after short-term use of haloperidol, lithium, and methimazole. Her thyroid function recovered and she was free from any psychiatric symptoms during the 1-year follow-up. Hyperthyroidism following surgery is not uncommon and its possibility should be considered when making differential diagnoses.