A pooled analysis of case-control studies of thyroid cancer. VI. Fish and shellfish consumption

Objective: To better understand the role of fish and shellfish on thyroid cancer risk, we systematically re-analyzed the original data from 13 case-control studies conducted in the US, Japan, China, and Europe. Methods: A total of 2497 cases (2023 women, 474 men) and 4337 controls (3268 women, 1069 men) were considered. Odds ratio (OR) and corresponding 95% confidence interval (CI) were estimated for each study by logistic regression models, conditioned on age and sex, and adjusted for history of goiter, thyroid nodules or adenomas, and radiation. Combined ORs were computed as the weighted average of the estimates from each study. Results: The ORs for the highest level of total fish consumption (three or more times per week) as compared to the lowest one (less than once per week) was above unity in Hawaii, Connecticut, Japan, Norway, Tromso, and Vaud. Conversely, the ORs for the studies in Los Angeles, Shanghai, southeastern Sweden, Uppsala, northern Sweden, northern Italy, and Athens were below one. The pattern of risk for salt water fish and shellfish was not substantially different from that of total fish. Fish was not associated with thyroid cancer risk in all studies combined (OR=0.99, 95% CI 0.85-1.2 for moderate, and OR=0.88, 95% CI 0.71-1.1 for high total fish consumption), but there was a suggestion of a protective effect in endemic goiter areas (OR=0.65, 95% CI 0.48-0.88). Conclusion: This combined analysis indicates that relatively elevated fish consumption does not appreciably increase thyroid cancer risk, and may have a favorable influence in areas where iodine deficiency is, or was, common.