Myxedema psychosis is uncommon in patients with primary hypothyroidism. Most often, this disease state can be found in patients with Hashimoto thyroiditis or after total thyroidectomy. Chronic hypothyroidism can lead to an insidious onset of psychiatric symptoms in patients, such as dementia, delirium, psychosis, hallucinations, and coma. A 31-year-old man with an unremarkable medical history was brought to the psychiatric emergency department for new-onset aggression, paranoid behavior, and hallucinations for 4 days. Initial test results showed a thyroid-stimulating hormone level of 306.0 mIU/L and a free thyroxin level of 0.24 ng/dL. No other clinical or laboratory abnormalities were found. A diagnosis of myxedema psychosis was established, and the patient was treated with tapering doses of intravenous hydrocortisone and 0.1 mg of intravenous levothyroxine daily. On hospital day 3, his mental status began to improve, and he was discharged on day 4. Myxedema psychosis is rarely the initial presenting symptom of hypothyroidism, especially in the absence of other abnormal clinical or laboratory findings.