['Clinical trials demonstrated that metformin increases the efficiency of systemic therapy in cancer patients.', 'We examined whether the efficacy of conventional treatment of differentiated thyroid cancer (DTC) is affected by therapy with metformin in diabetic patients.', 'We compared the rate of complete response (CR) between diabetics who were treated with metformin (group MF+; n = 34) or not treated (group MF-; n = 21) and control nondiabetic patients (group C; n = 185). We also examined the effects of metformin on DTC cells in vitro.', "The groups were comparable in terms of age, sex, body mass index, diabetes management, frequencies of multifocal tumor growth, extrathyroidal extension, and locoregional and distant metastases. Tumor size was significantly smaller in the MF+ group compared with the MF- and C groups (1.37 ± 0.97 vs 2.44 ± 1.49 vs 2.39 ± 1.73 cm, respectively; P = .026). A multivariate model revealed that extrathyroidal extension (P = .018), distant metastases (P < .0001), and lack of treatment with metformin of diabetics (P < .0001) decreased the likelihood of CR. A Cox hazards model revealed that age (P = .025), locoregional metastases (P = .022), distant metastases (P = .003), and lack of treatment with metformin of patients with diabetes (P = .014) are associated with increased risk for shortened progression-free survival. In vitro data revealed that metformin inhibited cancer cell growth, activated cAMP-inducible protein kinase (5'-AMP-activated protein kinase [AMPK]), and down-regulated p70S6K/pS6. Metformin potentiated H₂O₂-inducible activation of AMPK but attenuated pERK and p70S6K. Tumors from MF+ patients demonstrated a lower level of phospho-p70S6K compared with the MF- group."] Is treatment with metformin associated with higher remission rate in diabetic patients with thyroid cancer?