Induction of oxidative stress events has been shown to be associated with lithium (Li) hypothyroidism induction. Metformin (MET) is a commonly used antidiabetic drug with multiple properties including antiproliferative activity, antioxidant potency, and is used in polycystic ovarian syndrome treatment. Here, in this study, we aimed to investigate the effect of different doses of MET on Li‐induced hypothyroidism for elucidating its mechanism of action. The obtained results demonstrated the oxidative stress reduction in thyroid tissues upon MET treatment. Besides this, the biochemical analysis revealed a significant reduction in T3 and TSH levels (down to 2 ng/ml and 0.05 µU/ml, respectively) in coordination with an observable reduction in T4 level (up to 2.1 ng/ml). Also, a significant reduction in Li‐related tissue damages including changes in the morphology and the size of follicles, rate of vascularity, detachment of follicular cells, inflammatory cells infiltration, and follicular cells hypertrophy and disruption was observed. Ultimately, regarding the significant improvement in thyroid tissues and valuable antioxidant activity determined in tissues treated with MET, it is concluded that MET co‐administration with Li can significantly reduce the negative effects of Li and enhance the efficacy of Li therapy.