Smoking, alcohol consumption, and the risk of thyroid cancer: a population-based Korean cohort study of 10 million people.

Background: The association between smoking, alcohol consumption, and thyroid cancer has been evaluated in observational studies, yet the results remain controversial. In the present investigation, we analyzed a longitudinal cohort study with representative data to determine the association between smoking, alcohol consumption, and thyroid cancer risk, allowing for risk modification due to age and sex.

Methods: From the Korean National Health Insurance database, .subjects aged ≥20 who participated in health screening program in 2009 were identified and followed-up till 2017. The adjusted hazard ratio (aHR) for the risk of thyroid cancer was estimated by Cox proportional hazard model Results: During a mean follow-up period of 8.33 ± 0.57 years, out of 9,699,104 participants, 89,527 (0.9%) were diagnosed with thyroid cancer. In comparison with those who never smoked, current smokers [aHR: 0.74, 95% confidence interval (CI): 0.72-0.76)] had a decreased risk of thyroid cancer even though ex-smokers (aHR: 0.98, 95% CI: 0.96-1.01) did not. There was no dose-response relationship when the participants were examined with regard to the daily amount of smoking, duration of smoking, and pack-years. A decrease in the risk of thyroid cancer was observed according to both the number of drinking episodes per week and the number of glasses per once drinking (HR: 0.81, 95% CI: 0.74-0.88 for drinks consumed 7 times/week and HR: 0.87, 95% CI: 0.82-0.92 for ≥ 15 glasses once drinking vs. nondrinkers). A certain degree of alcohol consumption seemed to provoke a decreased risk of thyroid cancer as compared to those who did not drink (HR: 0.85, 95% CI: 0.81-0.89 for ≥ 40 g/week). The interaction of smoking and alcohol consumption was found to be significant (p-interaction < 0.0001), and sub-multiplicative.

Conclusions: We found that the risk of thyroid cancer incidence was inversely associated with smoking and alcohol consumption. A sub-multiplicative interaction between smoking and alcohol intake on the risk of thyroid cancer was found. Further studies are needed to elucidate exactly how smoking and alcohol are related to the pathogenesis and the modification their effects may have on thyroid cancer development.