Introduction: Undiagnosed thyroid dysfunction may impair metabolic control in patients with diabetes. Due to the lack of adequate studies on the incidence of thyroid dysfunction in patients with type 2 diabetes mellitus there is no consensus regarding optimal thyroid screening strategies in routine diabetes care. The aim of this study was to determine the incidence of thyroid dysfunction in patients with type 2 diabetes. Materials and Methods: In this study, patients aged≥30 years, participants of in the prospective Tehran Lipid and Glucose Study, who had complete thyroid assessments were divided into two groups of 428 diabetic and 2847 nondiabetic controls and followed for 12 years. The incidence of thyroid dysfunction in these two groups was compared and the relative risk of type 2 diabetes for thyroid dysfunction was calculated. Results: Compared to those without diabetes the disorders patients with diabetes were less likely to develop thyroid dysfunction (incidence rate 12.56 vs. 16.50 cases per 1000 person-years, Relative risk 0.74, 95% CI, 0.534-1.026, P=0.071). Conclusion: The incidence of thyroid dysfunction in type 2 diabetic patients is not higher than in non-diabetic individuals, indicating that routine screening of thyroid dysfunctions is not recommended in type 2 diabetic subjects.