SummaryTwo cases are presented in which treatment with lithium was complicated by thyrotoxicosis. The underlying physiological mechanisms are discussed.Lithium suppresses thyroid function, so the early stages of thyrotoxicosis may be clinically undetectable. The symptoms and signs of thyrotoxicosis may be confused with the side effects of lithium, and if lithium is withdrawn severe rebound thyrotoxicity may occur. Furthermore, some of the side effects of lithium may be aggravated by thyrotoxicity.These cases provide further evidence of the importance of routine screening of thyroid function before starting treatment with lithium.