Iodine and thyroid cancer risk among women in a multiethnic population: The Bay Area thyroid cancer study

Research on the relationship between iodine exposure and thyroid cancer ris k is limited, and the findings are inconclusive. In most studies, fish/shel lfish consumption has been used as a proxy measure of iodine exposure. The present study extends this research by quantifying dietary iodine exposure as well as incorporating a biomarker of long-term (I year) exposure, i.e., from toenail clippings. This study is conducted in a multiethnic population with a wide variation in thyroid cancer incidence rates and substantial di versity in exposure. Women, ages 20-74, residing in the San Francisco Bay A rea and diagnosed with thyroid cancer between 1995 and 1998 (1992-1998 for Asian women) were compared with women selected from the general population via random digit dialing. Interviews were conducted in six languages with 6 08 cases and 558 controls. The established risk factors for thyroid cancer were found to increase risk in this population: radiation to the head/ neck [odds ratio (OR), 2.3; 95% confidence interval (CI), 0.97-5.5]; history of goiter/nodules (OR, 3.7; 95% CI 2.5-5.6); and a family history of prolifer ative thyroid disease (OR, 2.5; 95% CI, 1.6-3.8). Contrary to our hypothesi s, increased dietary iodine, most likely related to the use of multivitamin pills, was associated with a reduced risk of papillary thyroid cancer. Thi s risk reduction was observed in "low-risk" women (i.e., women without any of the three established risk factors noted above; OR, 0.53; 95% CI, 0.33-0 .85) but not in "high-risk" women, among whom a slight elevation in risk wa s seen (OR, 1.4; 95% CI, 0.56-3.4). However, no association with risk was o bserved in either group when the biomarker of exposure was evaluated. In ad dition, no ethnic differences in risk were observed. The authors conclude t hat iodine exposure appears to have, at most, a weak effect on the risk of papillary thyroid cancer.