Increased cardiovascular risk in thyroid cancer patients taking levothyroxine: a nationwide cohort study in Korea

Objective Many thyroid cancer patients are exposed to long-term thyroid-stimulating hormone (TSH) suppression, often as lifetime treatment, and are consequently at risk for cardiovascular disease. We investigated the incidence of coronary heart disease (CHD) and ischemic stroke among thyroid cancer patients compared with matched control subjects. Design Retrospective cohort study. Methods A total of 182 419 subjects who received thyroidectomy for thyroid cancer during 2004-2012 were selected from the Korean National Health Insurance data, which cover approximately 97% of the entire Korean population. Propensity score matching was used to select non-cancer controls. Cox proportional hazards regression analysis was used to determine relative risk of coronary heart disease and ischemic stroke. Mean follow-up was 4.32 years. Results Thyroid cancer patients had elevated risk for CHD and ischemic stroke with hazard ratio (HR) of 1.15 (95% confidence interval (CI): 1.10-1.22) and 1.15 (1.09-1.22), respectively. This risk was increased in those who took a higher dosage of levothyroxine (HR: 1.47, 95% CI: 1.34-1.60 for CHD and HR: 1.56, 95% CI: 1.42-1.72 for ischemic stroke among those who took ≥170 μg/day levothyroxine). Although risk of atrial fibrillation was dose-dependently associated with levothyroxine dosage, it represents only a small proportion of ischemic stroke incidence (4.4%, 128/2914). Conclusions The risk for CHD and ischemic stroke was higher in thyroid cancer patients who received thyroidectomy, and the dosage of levothyroxine administered appears to play a major role. Greater caution is suggested for the screening and treatment of thyroid cancer and subsequent TSH suppression therapy, as well as proper management for cardiovascular disease prevention.