Smoking, Alcohol Consumption, and the Risk of Thyroid Cancer: A Population-Based Korean Cohort Study of 10 Million People

Background: It is unclear if cigarette smoking and alcohol consumption are associated with thyroid cancer risk. Our aim was to explore for any associations between cigarette smoking and alcohol consumption with thyroid cancer, after adjusting for potential confounders. Methods: Using data from the Korean National Health Insurance database, we retrospectively identified individuals aged ≥20 years who participated in the 2009 health screening program and were followed until 2017. We estimated the adjusted hazard ratio (aHR) for the risk of thyroid cancer using a Cox proportional hazard model, adjusted for age, sex, regular exercise, monthly income, body mass index, diabetes mellitus, and dyslipidemia. Results: During a mean follow-up period of 8.33 ± 0.57 years, of 9,699,104 participants, 89,527 (0.9%) were diagnosed with thyroid cancer. Compared with those who never smoked, current smokers had a lower risk of thyroid cancer (aHR: 0.74, 95% confidence interval [CI]: 0.72-0.76), while ex-smokers did not (aHR: 0.98, 95% CI: 0.96-1.01). There was no significant dose-response relationship with regard to daily amount smoked, duration of smoking, or pack-years. A reduced risk of thyroid cancer was observed in subjects who reported the following categories of alcohol intake (compared with none): mild (aHR: 0.92, 95% CI: 0.90-0.93), moderate (aHR: 0.86, 95% CI: 0.84-0.89), and heavy (aHR: 0.86, 95% CI: 0.82-0.89). Inverse associations with thyroid cancer risk were observed regarding the number of drinking episodes per week and the number of drinks per occasion. A submultiplicative effect of smoking and alcohol consumption was observed (p-interaction <0.001). Conclusions: We observed that thyroid cancer risk was inversely associated with smoking and alcohol consumption, with a significant interaction between these variables.