Objective: To investigate the changes of thyroid function in patients with type 2 diabetes mellitus. Methods: A retrospective study in serum thyroid function was carried out among 805 diabetic patients in Anhui provincial hospital and 226 controls. Results: 1) The prevalence of type 2 diabetes patients with abnormal thyroid function was 20.12% and their respective prevalence were: hyperthyroidism (1.99%), subclinical hyperthyroidism (2.86%), hypothyroidism (2.24%), subclinical hypothyroidism (13.04%); The prevalence of abnormal thyroid function and subclinical hypothyroidism in type 2 diabetes patients were significantly higher than those in the normal group (10.18% and 4.87%, respectively(P<0.05); The prevalence of thyroid dysfunction and subclinical hypothyroidism in female patients were significantly higher than that in males (P<0.01); 2) The prevalence of subclinical hypothyroidism increased with the increasing age in type 2 diabetes, but the association of which with the duration of diabetes was not found. The incidence of subclinical hypothyroidism in antibody positive (TPOAb or TGAb was positive) patients was significantly higher than that in negative patients (TPOAb and TGAb were negative). Conclusion: Prevalence of abnormol thyroid function is quite high in type 2 diabetes mellitus, especially in female and patients above 55 years, who we suggest thyroid funcion should be checked out in.