Obesity is positively related and tobacco smoking and alcohol consumption are negatively related to an increased risk of thyroid cancer.

The purpose of this study was to evaluate the relationships of smoking, alcohol consumption, and obesity with thyroid cancer in Korean residents. The Korean National Health Insurance Service-Health Screening Cohort includes individuals ≥ 40 years who were assessed from 2002 to 2013. In total, 4977 thyroid cancer participants were matched with respect to age, sex, income, and region of residence with 19,908 controls at a ratio of 1:4. Crude and adjusted (for the Charlson comorbidity index, smoking status, frequency of alcohol consumption, and obesity) odds ratios (ORs) were analyzed using conditional logistic regression analyses. Additionally, 95% confidence intervals (CIs) were calculated. The adjusted OR of smoking for thyroid cancer was 0.62 (95% CI 0.54-0.72, P < 0.001), and that of alcohol consumption was 0.83 (95% CI 0.75-0.92, P < 0.001). The adjusted ORs of the BMI categories were 1.13 (95% CI 1.05-1.22, P = 0.002) for obese I, and 1.24 (95% CI 1.04-1.47, P = 0.014) for obese II. The ORs of smoking and alcohol consumption were lower, and those of overweight and obesity were higher in thyroid cancer patients than in individuals in the control group.