Association Between Antibiotic Exposure and Thyroid Cancer: A Nationwide Cohort Study in South Korea.

Background: Although recent studies have introduced antibiotics as a potential risk factor for thyroid cancer, further studies are necessary. We examined the association between long-term antibiotic usage and thyroid cancer risk. Methods: This nationwide cohort study investigated 9,804,481 individuals aged 20 years or older who participated in health screening (2005-2006) with follow-up ending on December 31, 2019, using the Korean National Health Insurance Service database. Multivariable Cox proportional hazards regression was used to estimate adjusted hazard ratios (aHRs) and 95% confidence intervals (CIs) for thyroid cancer risk according to the cumulative days of antibiotic prescription and the number of antibiotic classes, respectively. A 1:1 propensity score (PS) matching was also performed for analysis. Results: Compared with nonusers of antibiotics, participants prescribed ≥365 days of antibiotics showed an increased risk of thyroid cancer (aHR, 1.71; CI, 1.66-1.78) after adjusting for covariates including age, smoking status, comorbidities including thyroid-related diseases, and the number of head and neck computed tomography scans. Participants prescribed ≥365 days of antibiotics also had a significantly increased risk of thyroid cancer (aHR, 1.37; CI, 1.34-1.40) compared with participants prescribed 1-14 days of antibiotics. Association remained significant in the 1:1 PS-matched cohort. Moreover, compared with nonusers of antibiotics, the 5 or more antibiotic class user group had a higher thyroid cancer risk (aHR, 1.71; CI, 1.65-1.78). Conclusions: Long-term antibiotic prescriptions and an increasing number of antibiotic classes may be associated with a higher risk of thyroid cancer in a duration-dependent manner. The effects of long-term antibiotic exposure on thyroid cancer should be further investigated.