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.
From the aDivision of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, Rockville, MD; bCenter for Health Effects of Environmental Contamination, University of Iowa, Iowa City, IA; cDivision of Epidemiology and Community Health, School of Public Health, University of Minnesota, Minneapolis, MN; and dDivision of Epidemiology, College of Medicine, Mayo Clinic, Rochester, MN.
Submitted 21 April 2009; accepted 20 October 2009.
Supported in part by the Intramural Research Program of the NIH/National Cancer Institute. The Iowa Women's Health Study was supported by grant R01 CA39742.
Correspondence: Mary H. Ward, Division of Cancer Epidemiology and Genetics, Occupational and Environmental Epidemiology Branch, National Cancer Institute, 6120 Executive Blvd, EPS 8006, Bethesda, MD 20892. E-mail: firstname.lastname@example.org.