Ionizing radiation is the only established risk factor for thyroid cancer. An international pooled analysis of 14 case-control studies of thyroid cancer (2,725 cases and 4,776 controls) was conducted to further describe the etiology of this cancer. By combining many small studies, investigators were able to explore many suspected risk factors for thyroid cancer with adequate statistical power. The pooled analysis identified goiter and benign nodules as the strongest risk factors apart from radiation. The large excess risk of thyroid cancer in females was not strongly associated with menstrual or reproductive factors. Cigarette smoking was associated with a reduced risk of thyroid cancer in both men and women. The analysis did not find any evidence for an increased risk of thyroid cancer with increased iodine intake from seafood or fish.
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