Research Article The Association between Type 2 Diabetes Mellitus and Thyroid Cancer Young-Gyun Seo, 1 Ho-Chun Choi, 2 Ah Reum An, 3 Do Joon Park, 4 Young Joo Park, 4 Kyu Eun Lee, 5 Sue K. Park, 6,7,8 Yunji Hwang, 6,7,8 and Belong Cho 3,9,10 1 Department of Family Medicine, Hallym University Sacred Heart Hospital, Anyang, Gyeonggi-do 14068, Republic of Korea 2 Department of Family Medicine, Healthcare System Gangnam Center, Seoul National University Hospital, Seoul 03080, Republic of Korea 3 Department of Family Medicine, Center for Health Promotion and Optimal Aging, Health Promotion Center for Cancer Survivor, Seoul National University Hospital, Seoul 03080, Republic of Korea 4 Department of Internal Medicine, Seoul National University College of Medicine, Seoul 03080, Republic of Korea 5 Department of Surgery, Seoul National University Hospital & College of Medicine, Seoul 03080, Republic of Korea 6 Department of Preventive Medicine, Seoul National University College of Medicine, Seoul 03080, Republic of Korea 7 Department of Biomedical Science, Seoul National University Graduate School, Seoul 03080, Republic of Korea 8 Cancer Research Institute, Seoul National University, Seoul 03080, Republic of Korea 9 Advanced Institutes of Convergence Technology, Seoul National University, Suwon, Gyeonggi-do 16229, Republic of Korea 10 Institute on Aging, Seoul National University College of Medicine, Seoul 03080, Republic of Korea Correspondence should be addressed to Belong Cho; belong@snu.ac.kr Received 15 April 2017; Revised 30 May 2017; Accepted 12 June 2017; Published 9 July 2017 Academic Editor: Toshiyasu Sasaoka Copyright © 2017 Young-Gyun Seo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Aim. The incidence of thyroid cancer is increasing worldwide. The prevalence of type 2 diabetes mellitus (T2DM) is also increasing. Therefore, we aimed to analyze the effect of T2DM on thyroid cancer. Methods. A case-control study was performed. A total of 415 healthy controls with thyroid ultrasound screening and physician consultation were selected from the Thyroid Cancer Longitudinal Study (T-CALOS). Among patients with thyroid cancer who were enrolled in T-CALOS, 415 patients were matched to the control group according to age and sex. We assessed the effects of T2DM, T2DM duration, and T2DM medication on thyroid cancer. Results. Women with T2DM had lower odds of thyroid cancer than women without T2DM (odds ratio [OR]: 0.40, 95% confidence interval [CI]: 0.20–0.81). Individuals receiving T2DM medication had higher odds of thyroid cancer compared to those without T2DM medication (OR: 5.21, 95% CI: 1.58–17.15). Individuals with T2DM duration <6 years had lower odds of thyroid cancer compared to those without T2DM (OR: 0.58, 95% CI: 0.34–0.97). Conclusions. Individuals with early T2DM are presumed to have a low incidence of thyroid cancer, and this effect seems to last up to 6 years after diagnosis of T2DM. 1. Introduction The incidence of thyroid cancer is increasing worldwide. The increase in the incidence of thyroid cancer is likely due to a combination of the increase due to more sensitive diagnostic procedures and the true increase due to increased exposure to radiation and to other yet unknown carcinogens [1, 2]. Because of a particularly significant increase in Korea [3], the Thyroid Cancer Longitudinal Study (T-CALOS) was initiated in 2010 to analyze the characteristics of thyroid cancer [4]. The incidence and prevalence of type 2 diabetes mellitus (T2DM) vary from country to country, but the overall trend is increasing prevalence of T2DM in all countries [5]. The prevalence of diabetes among adults 30 years or older is 13.7% and more than 30% of persons aged ≥65 years have diabetes in Korea in 2014 [6]. Hindawi Journal of Diabetes Research Volume 2017, Article ID 5850879, 8 pages https://doi.org/10.1155/2017/5850879