Quetiapine is an atypical antipsychotic drug with a benign side effect profile. However, recent studies have reported that thyroid dysfunction is associated with quetiapine treatment. The authors report a patient with DSM-IV bipolar I disorder who developed subclinical hypothyroidism during quetiapine treatment. The patient showed no significant clinical symptoms, but only abnormal thyroid function test findings including antithyroglobulin antibody. The abnormal thyroid function test findings were normalized after discontinuation of quetiapine. The subclinical hypothyroidism developed during quetiapine treatment may be associated with autoimmune process.