Apathetic hyperthyroidism is an uncommon presentation of thyrotoxicosis that is most commonly observed in elderly individuals. A small number of cases, however, have been observed in young adults-all of whom were female. We describe a case of apathetic hyperthyroidism due to painless thyroiditis in a 23-year-old active duty male, who presented with a 1-month history of change in behavior characterized by poverty of speech and disinterest in his surrounding environment. Initial evaluation for neuropsychological disorders was negative. Thyroid function tests showed a thyrotoxic state. Results of thyroid antibody testing and a thyroid uptake and scan supported the diagnosis of painless thyroiditis. The patient's presenting symptoms were attributed to apathetic thyrotoxicosis and he was treated supportively. His speech and social involvement slowly improved. Although uncommon in young adults, we recommend that apathetic hyperthyroidism be considered in the differential diagnosis of withdrawn social behavior and subsequently investigated with thyroid function tests.