Relationship Between Physical Activity Levels and Thyroid Cancer Risk: A Prospective Cohort Study in Korea.

Physical activity is a protective factor against several types of cancers. However, evidence for the association between physical activity and thyroid cancer (TC) is still inconclusive. We used prospectively collected data from the Korea National Cancer Screenee Cohort, which consisted of 30,435 participants from 20 years who received health examinations at National Cancer Center between June 2007 and December 2014. Participants' follow-up data up to December 2019 was used to identify new TC cases. Demographic characteristics of the subjects were collected using a self-administered questionnaire. Physical activity measurement was analyzed from 15,175 participants using International Physical Activity Questionnaire-Short Form. Physical activity data included frequency (days per week) and duration (minutes per day) of their exercises in three intensity levels (walking, moderate, and vigorous-intensity). The association between physical activity levels and TC risk was examined by Cox proportional hazards regression models. We identified 234 new TC cases among 15,175 eligible participants during the follow-up period. Participants with the highest physical activity level had a reduced risk of TC (hazard ratio [HR] = 0.65 [confidence interval, CI = 0.44-0.94], -trend = 0.028) than participants with the lowest physical activity level. The significant associations were stronger among female subjects with a body mass index ≥25 kg/m (HR = 0.38 [CI = 0.16-0.93], -trend = 0.034), subjects with household income >4 million won/month (HR = 0.53 [CI = 0.30-0.94], -trend = 0.034), subjects without a first-degree family history of TC (HR = 0.66 [CI = 0.45-0.96], -trend = 0.040), and subjects who did not drink alcohol (HR = 0.48 [CI = 0.26-0.88], -trend = 0.018) or smoke (HR = 0.61 [CI = 0.40-0.95], -trend = 0.030). This prospective Korean cohort study suggests that increased physical activity may be protective for development of TC. These findings require confirmation in other populations.