Smoking and alcohol consumption in relation to risk of thyroid cancer in postmenopausal women

Background: Few cohort studies have examined smoking and alcohol consumption in relation to risk of thyroid cancer, and their findings are conflicting. Methods: We therefore assessed the association of smoking and alcohol intake with risk of thyroid cancer in a cohort of 159,340 women enrolled in the Women's Health Initiative. Over 12.7 years of follow-up 331 cases of thyroid cancer, of which 276 were papillary thyroid cancer, were identified. Cox proportional hazards models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI). Results: Compared to never smokers, ever smokers did not have altered risk. Current smokers had reduced risk for all thyroid cancer (HR 0.54, 95% CI 0.29-1.00) and for papillary thyroid cancer (HR 0.34, 95% CI 0.15-0.78); however, the number of current smokers among cases was small. No associations or trends were seen for amount smoked, age of starting smoking, or age at quitting. Smokers of ≥40 pack-years had a significantly reduced risk of papillary thyroid cancer (HR 0.44, 95% CI 0.21-0.89). In contrast, women who had smoked for < 20 years had increased risk of thyroid cancer (HR 1.35, 95% CI 1.05-1.74) and papillary cancer (HR 1.43, 95% CI 1.09-1.89). Alcohol intake was not associated with risk. Conclusion: Our findings suggest that current smoking and having higher pack-years of exposure are associated with a modestly reduced risk of thyroid cancer, whereas alcohol consumption does not appear to affect risk.