Risk Factors for Thyroid Cancer in Korea : A Prospective Cohort Study

Abstract

Recently, attention has been given to an increasing trend of thyroid cancer in many countries. An increasing trend of thyroid cancer has also been observed previously in Korea. There are indications that many potential risk factors are causally related to the risk of thyroid cancer, but data are not consistent. The purpose of the present study was to investigate demographic, anthropometric and lipid profile risk factors for thyroid cancer and to clarify the reasons for the exceptionally high incidence, using a large case, population-based, prospective cohort study. This study provided a unique opportunity to identify new clues to the etiology of this type of cancer. The study population included 268,947 participants (157,997 men and 110,950 women), aged 20-70 years, who underwent a health examination between 1994 and 2001 at sixteen hospitals in Korea and were followed from 1994 to 2007. Cox proportional hazards models were used to evaluate the association between each of the potential risk factors and thyroid cancer. Among the 268,947 subjects, there were 1,405 incident cases of thyroid cancer (347 men and 1,058 women) during the median 7.99 years of follow up. We found that thyroid cancer incidence rates significantly increased slightly from 1994 to 2007. There was a 3-4-fold higher incidence among women relative to men, and women exhibited a relatively rapid increase in incidence rates in the under 50 aged group and with a peaked in the 50-59 age group. The present study demonstrated that high height and weight were significantly associated with an increased risk of thyroid cancer, whereas no physical exercise was associated with decreased risk of thyroid cancer in both genders. In addition, women with high education levels, a past history of thyroid disease and family history of thyroid cancer had increased risk for thyroid cancer, while current smoking

was inversely associated for men. There were no clear associations between lipid profile factors and the risk of thyroid cancer. In conclusion, this population-based, prospective cohort study suggests that high height and weight, education levels, past history of thyroid disease and family history of thyroid cancer were associated with an increased risk of thyroid cancer, while no physical exercise and current smoking were inversely associated. These findings may be helpful for understanding the mechanisms involved in the development of thyroid cancer. Further investigations need to determine the risk factors for thyroid cancer incidence.