Objective: Hypothyroidism is a well-documented consequence of lithium treatment. Less well known is a possible association between lithium therapy and hyperthyroidism. This may have clinical implications as rapid changes in thyroid hormones may worsen a person's affective state, while symptoms of hyperthyroidism can mimic those of mania. We therefore systematically reviewed the published literature for evidence of lithium-induced hyperthyroidism. Methods: We searched PubMed, Embase and CINAHL for articles where individuals developed biochemically confirmed hyperthyroidism (with or without clinical symptoms), while on lithium therapy for an affective illness. We included case reports, case series, cross-sectional, case control and cohort studies. Results: We included 52 studies, 39 of which were individual case reports and 3 were case series. There were 10 crosssectional or case control or cohort studies. All the research designs suggested an association between the prescription of lithium and hyperthyroidism. However, these findings were limited by the quality of the included studies, small number of participants and the general lack of either a clear temporal relationship or dose response. Conclusion: Hyperthyroidism is an uncommon side-effect of lithium compared to hypothyroidism but may have clinical implications. However, large prospective studies are required to clarify this association and to further inform the management of patients treated with lithium where hyperthyroidism occurs.