Dietary iodine and thyroid cancer risk in French Polynesia: a case-control study.

Background: French Polynesia has one of the world's highest thyroid cancer incidence rates. Iodine is suspected to play a role in this high incidence. The objective of this study was to assess whether low dietary iodine is related to a higher risk of thyroid cancer in the French Polynesian population.
Methods: A case-control study was performed among native residents of French Polynesia. It included 229 cases of differentiated thyroid cancer diagnosed between 1979 and 2004 (203 women, 26 men) matched with 371 population controls (324 women, 47 men) on the date of birth. The current study is focused on dietary iodine intake and fish consumption (food rich in iodine) and analyzed by conditional logistic regression.
Results: Daily dietary iodine intake was insufficient (<150 μg/day) in 60% of both cases and controls. A decreased risk of thyroid cancer was observed with a higher consumption of fish (p(trend)=0.008) and shellfish (p(trend)=0.002), and also with a higher dietary iodine intake (p(trend)=0.03). There was no significant interaction between the effects of the thyroid radiation dose and the dietary iodine intake (p=0.2).
Conclusion: French Polynesia is a mild iodine deficiency area in which a higher consumption of food from the sea and a higher dietary iodine intake are significantly associated with a decreased risk of thyroid cancer. The quantification of this reduction requires specific investigation of iodine intake in traditional Polynesian food.