INTRODUCTION Thyroid cancer incidence in Korea is the highest in the world and has recently increased steeply. However, factors contributing to this sudden increase have not been fully elucidated, and few studies have explored the postoperative prognosis. The Thyroid Cancer Longitudinal Study (T-CALOS) was initiated with three aims: (1) to identify factors predicting quality of life, recurrence, and incidence of other diseases after thyroid cancer treatments; (2) to investigate environmental exposure to radiation, toxicants and molecular factors in relation to tumour aggressiveness; and (3) to evaluate gene-environment interactions that increase thyroid cancer in comparison with healthy participants from a pool of nationwide population-based healthy examinees.

METHODS AND ANALYSIS T-CALOS enrols patients with incident thyroid cancer from three general hospitals, Seoul National University Hospital, Seoul National University Bundang Hospital and National Medical Center, Korea. The study is an ongoing project expecting to investigate 5000 patients with thyroid cancer up until 2017. Healthy examinees with a normal thyroid confirmed by sonography have been enrolled at the Healthy Examination Center at Seoul National University Hospital. We are also performing individual matching using two nationwide databases that are open to the public. Follow-up information is obtained at patients' clinical visits and by linkage to the national database. For statistical analysis, we will use conditional logistic regression models and a Cox proportional hazard regression model. A number of stratifications and sensitivity analyses will be performed to confirm the results.

ETHICS AND DISSEMINATION Based on a large sample size, a prospective study design, comprehensive data collection and biobank, T-CALOS has been independently peer-reviewed and approved by the three hospitals and two funding sources (National Research Foundation of Korea and Korean Foundation for Cancer Research). The results of T-CALOS will be published according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) criteria.