Incidence of hypothyroidism after treatment for breast cancer: A Korean population-based study.

This Korean population-based study aimed to describe the patterns of hypothyroidism after adjuvant radiation therapy (RT) in patients with breast cancer. The Korean Health Insurance Review and Assessment Service database was searched for patients with invasive breast carcinomas. We calculated the cumulative incidence and incidence rates per 1,000 person-years of subsequent hypothyroidism and compared them using the log-rank test and the Cox proportional hazards model. Between 2007 and 2018, 117,135 women diagnosed with breast cancer with a median follow-up time of 4.6 years were identified. The 8-year incidence of hypothyroidism was 9.3% in patients treated with radiation and 8.6% in those treated without radiation (p = 0.002). The incidence rates per 1,000 person-years in the corresponding treatment groups were 6.2 and 5.7 cases, respectively. The hazard ratio (HR) in patients receiving RT was 1.081 (95% confidence interval [CI], 1.013-1.134; p = 0.002). After mastectomy, RT showed a trend toward a higher risk of hypothyroidism (HR = 1.248; 95% CI, 0.977-1.595; p = 0.076). Our study provides one of the largest population-based data analyses regarding the risk of hypothyroidism among Korean patients with breast cancer. The adjusted risk for patients treated with RT exceeded that for patients with breast cancer treated without RT. The effect was evident immediately after treatment and lasted up to approximately 9 years.