Objective: Abnormal thyroid function after thyroidectomy and subsequent thyroid-stimulating hormone suppression can have detrimental effects on glucose homeostasis in thyroid cancer patients. We aimed to investigate whether thyroidectomy increases the risk of type 2 diabetes in thyroid cancer patients and to explore the association between levothyroxine dosage and type 2 diabetes risk. Methods: From the Korean National Health Insurance database, we included 36,377 thyroid cancer patients without known diabetes who underwent thyroidectomy between 2004 and 2013. Matched non-thyroid cancer subjects were selected using 1:1 propensity score matching. The main outcome was the incidence of type 2 diabetes mellitus. Results: Thyroid cancer patients who underwent thyroidectomy had a higher risk of developing type 2 diabetes mellitus than the matched controls (hazard ratio [HR]: 1.43, 95% confidence interval [CI]: 1.39-1.47). Among thyroid cancer patients, when the second quartile group (in terms of the mean levothyroxine dosage) was considered the reference group, the risk of type 2 diabetes mellitus increased in the first quartile (HR: 1.45, 95% CI: 1.36- 1.54) and fourth quartile groups (HR: 1.37, 95% CI: 1.29-1.45); meanwhile, the risk decreased in the third quartile group (HR: 0.91, 95% CI: 0.85-0.97). Conclusion: Thyroid cancer patients who underwent thyroidectomy were more likely to develop type 2 diabetes mellitus than the matched controls. There was a U-shaped dose-dependent relationship between the levothyroxine dosage and type 2 diabetes mellitus risk. Therefore, thyroid cancer patients with a history of thyroidectomy should be screened for type 2 diabetes.