OBJECTIVE To report a case of apathetic thyrotoxicosis with an etiology of subacute thyroiditis. METHODS We describe the patient's clinical findings, laboratory findings, and clinical course. RESULTS An 85-year-old woman with no history of thyroid disease presented with severe obtundation and altered mental status. Laboratory testing documented elevated free thyroxine and free triiodothyronine concentrations and a suppressed thyrotropin concentration. Thyroid antibodies were absent. A radioactive iodine study revealed severely diminished uptake, suggestive of thyroiditis. After a short course of steroids, the patient's mental status returned to baseline. Follow-up laboratory testing showed normalizing thyroid function. CONCLUSIONS Even in the absence of a history of thyroid disease, we recommend considering thyroid dysfunction in the differential diagnosis of patients who present with altered mental status, particularly in the elderly population.