Abnormalities in thyroid function are associated with many psychiatric symptoms. We present a report of a 15-year-old girl who was admitted to the psychiatry inpatient unit with symptoms of suicidal behavior, irritability, and impulsivity. One year previously, she had become more short-tempered, and had started to cut her wrists impulsively. Laboratory tests revealed subclinical hyperthyroidism. She was treated with anxiolytic and antithyroid drugs, and her suicidal ideation and irritability resolved. This case demonstrates that subclinical hyperthyroidism can be associated with suicidal behavior as well as overt hyperthyroidism. Early intervention is required to prevent suicidal behavior in patients with subclinical hyperthyroidism.