Objective To investigate the effect of high serum thyroid stimulating hormone (TSH) on malignant transformation of thyroid tissue in Hashimoto's thyroiditis (HT).Methods The data of 569 patients pathologically confirmed HT admitted from Jan.2009 to Dec.2013 were retrospectively analyzed.According to with or without papillary thyroid carcinoma (PTC),patients were divided into HT group and HT complicated with PTC group.Age,gender,preoperative TSH,free thyroxine,free tri-iodothyronine,thyroidperoxidaseantibody (TPOAb),antithyroglobulinanfibody (TGAb),and tumor multifocality were investigated.Results Among the 569 patients,306 patients had HT without PTC and 263 had HT with PTC.The avaerage age of patients complicated with PTC was (45.52±10.58) years,younger than that of patients without PTC (51.63+9.50) years (P＜0.05).In HT patients with or without PTC,31％ (81/263) and 22.5％ (69/306) of patients had high TSH levels,which was significantly different (P＜0.05).Among HT patients complicated with PTC,tumor multifocality was identified in 32％(26/81) of patients with high level of TSH and in 29％(52/182) of patients with normal level of TSH.The difference had no statistical significance (P＞0.05).Conclusion High preoperative TSH level may stimulate malignant transformation of thyroid tissue in HT patients.