We evaluated the correlation between serum thyroid-stimulating hormone (TSH) levels and tumor size and other invasiveness parameters of tumor in patients with differentiated thyroid carcinoma (DTC). Several clinical studies have reported that TSH may also have a role as a regulator of the development and function of the thyroid gland. It is currently not clear whether TSH is involved in the existence of thyroid cancer or progression of thyroid cancer or both. Patients with DTC who underwent thyroid surgery between 2003 and 2008 were included this study. Preoperative serum T3, T4, and TSH levels were compared with the size and invasiveness of cancer, retrospectively. DTC was observed in 110 patients over the 5-year period. Seventy-seven (70 %) of them were euthyroid and classified as the "normal-TSH group" (NTG), and 33 (30 %) have an overt or subclinical hyperthyroidism, classified as the "low-TSH group" (LTG). The mean tumor diameter in the LTG was found to be 8.91 +/- 8.03 mm; however, it was found to be 18.19 +/- 16.24 mm in the NTG. There were significantly differences among the groups related to the diameter of tumor (p=0.001). Microcarcinoma was determined in 36 patients (46.8 %) in the NTG and 23 patients (69.7 %) in the LTG (p=0.027). Although there were no significant differences, tumor capsule invasion (33.8 vs. 18.2 %, p=0.099) and lymphovascular invasion (16.9 vs. 6.1 %, p=0.130) rates were higher in the NTG. These findings suggest that TSH has effects on growing and proliferation of not only normal thyroid cells but also cancer cells in DTC. This study revealed that serum TSH level can be explored as an important factor that affects the size and invasiveness of tumor in DTC.