Differentiated thyroid cancer is the most common neoplasm of endocrine system. After surgery and radioiodine treatment, thyroid stimulating hormone (TSH) suppression is the main goal which is achieved with levothyroxine treatment. Levothyroxine causes increased thyroid hormones which can have negative impact on bone and cardiovascular system. Anecdotal reports have shown that metformin can induce TSH suppression without change in T3 and T4 concentration. The purpose of this study was to prescribe metformin as additional drug to levothyroxin in order to decrease levothyroxine dosage.