Risk factors for thyroid cancer : an epidemiological review focused on nutritional factors

Objectives: The present review summarizes epidemiological evidence on risk factors for thyroid cancer (TC), in particular, nutritional factors. Methods: Searches of articles on the issue were conducted using MEDLINE. Results: Exposure to ionizing radiation, particularly during childhood, is the best-established risk factor for TC. There is also a strong association with history of benign nodules/adenoma or goiter. Iodine deficiency may induce an increasing incidence of benign thyroid conditions, but very high iodine intake also affects thyroid function and, possibly, TC risk. Among dietary factors, fish-the major natural source of iodine in human diet-is not consistently related to TC risk. High intake of cruciferous vegetables shows a weak inverse association with TC. Among other food groups, vegetables other than cruciferous are the only food group showing a favorable effect on TC, with an approximate 20% reduction in risk for subjects with the highest consumption. No effect on TC risk of alcohol, coffee, or other food-groups/nutrients emerged. Height and weight at diagnosis show a moderate positive association with TC risk. Conclusion: At present, the only recognized measures for reducing TC risk is to avoid ionizing radiation and iodine deficiency, particularly in childhood and young women, and to increase vegetable consumption.