Nitrate From Drinking Water and Diet and Thyroid Disorders and Thyroid Cancer Among Women

Abstract

ISEE-698 Objective: At high concentrations, nitrate competitively inhibits iodide uptake by the thyroid. In previous studies, elevated nitrate levels in drinking water supplies were associated with increased thyroid volume among women and subclinical hypothyroidism among children in the Netherlands and Slovakia, respectively. Nitrate intake has not been evaluated as a risk factor for thyroid cancer. Materials and Methods: We evaluated nitrate intake from drinking water and diet among a cohort of 21,977 Iowa women who were 55 to 69 years of age at baseline in 1986 and used a public or private water supply for >10 years (87% >20 years). There were 45 incident thyroid cancers through 2005. At enrollment, the number of women reporting that they ever took medication for hyper- or hypothyroidism were 1009 (4.6%) and 3151 (14.3%), respectively. We calculated the number of years the women used a public supply with ≥ 5 mg/L nitrate-nitrogen from 1955 to 1988 and the average nitrate level over this period. Dietary nitrate was assessed using a 126-item food frequency questionnaire. Results: Five or more years using a public water supply ≥ 5 mg/L nitrate-nitrogen was associated with an increased risk of thyroid cancer [adjusted Relative Risk (RR) = 2.40, 95% Confidence Interval (CI) 0.97–5.92] and hypothyroidism (RR = 1.22, 95% CI 1.09–1.36) compared to no years of exposure at this level; there was no association for hyperthyroidism. Dietary nitrate, which came primarily from vegetable intake, was associated with an increased risk of hypothyroidism and total nitrate from dietary and drinking water sources showed the strongest association (highest quartile RR = 1.38, 95% 1.23–1.55); dietary and total nitrate were not associated with thyroid cancer or hyperthyroidism. Conclusions: Further investigations in other populations are necessary to confirm these findings, which have potential public health significance due to widespread exposure to elevated nitrate levels in drinking water supplies.