Nitrate Intake and the Risk of Thyroid Cancer and Thyroid Disease


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Abstract

Background:Nitrate is a contaminant of drinking water in agricultural areas and is found at high levels in some vegetables. Nitrate competes with uptake of iodide by the thyroid, thus potentially affecting thyroid function.Methods:We investigated the association of nitrate intake from public water supplies and diet with the risk of thyroid cancer and self-reported hypothyroidism and hyperthyroidism in a cohort of 21,977 older women in Iowa who were enrolled in 1986 and who had used the same water supply for >10 years. We estimated nitrate ingestion from drinking water using a public database of nitrate measurements (1955–1988). Dietary nitrate intake was estimated using a food frequency questionnaire and levels from the published literature. Cancer incidence was determined through 2004.Results:We found an increased risk of thyroid cancer with higher average nitrate levels in public water supplies and with longer consumption of water exceeding 5 mg/L nitrate-N (for ≥5 years at >5 mg/L, relative risk [RR] = 2.6 [95% confidence interval (CI) = 1.1–6.2]). We observed no association with prevalence of hypothyroidism or hyperthyroidism. Increasing intake of dietary nitrate was associated with an increased risk of thyroid cancer (highest vs. lowest quartile, RR = 2.9 [1.0–8.1]; P for trend = 0.046) and with the prevalence of hypothyroidism (odds ratio = 1.2 [95% CI = 1.1–1.4]), but not hyperthyroidism.Conclusions:Nitrate may play a role in the etiology of thyroid cancer and warrants further study.

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