A 32-year-old woman with rapid cycling bipolar illness had numerous clinical problems throughout 19 affective episodes and six hospital admissions within 34 months. Persistent hyperthyroxinemia, always associated with manic episodes, led to a diagnostic work-up that ruled out a primary thyroid dysfunction and pointed to oral contraceptives, appetite suppressants, and psychiatric illness as likely causes of elevated T4 values. The contention that an underlying thyroid hypofunction is the basis of rapid cycling is questioned. The extent to which a bipolar disorder increases T4 levels and the role of euthyroid hyperthyroidinemia in the pathogenesis of rapid cycling are discussed.