Dietary Factors and the Risk of Thyroid Diseases: A Review

ABSTRACT

BACKGROUND AND OBJECTIVES: Diet is one of the major risk factors for thyroid diseases. It has been shown that high or excessive iodine intake is more likely to be a health concern in iodine-sufficient regions or regions where iodine deficiency previously existed due to the emergence of iodine-induced hypothyroidism or hyperthyroidism. Therefore, this review investigates the occurrence of thyroid diseases, and particularly hypothyroidism and hyperthyroidism, in populations with different levels of iodine intake and other dietary factors in various geographic regions. MATERIALS AND METHODS: A total of 856 articles published between January 1st, 1990 and March 31st, 2015, were identified. Epidemiological studies that showed an association between dietary factors and thyroid diseases were selected, yielding a total of 21 articles. RESULTS: Due to a sudden increase in iodine supplementation (i.e., via salt iodization), regions such as Denmark and China, where insufficient iodine intake previously existed, showed a significant increase in the occurrence of hypothyroidism compared with that of hyperthyroidism. Other dietary factors, such as nitrate intake, may increase the risk of the diseases, whereas a vegan diet and alcohol intake may lower the risk. CONCLUSION: The level of iodine intake is quite variable between individuals in different geographic regions, and the risk of thyroid diseases may also vary by age and gender. Therefore, monitoring of safe levels of iodine intake should be performed to prevent iodine-induced thyroid diseases.