Nitrates in drinking water and food increase risk of thyroid cancer and thyroid hormone disease.

Long-term exposure to nitrates through food and water may increase a woman’s risk of thyroid disease, finds a study of older women in Iowa. Public water supplies contaminated with nitrates increased the risk of thyroid cancer in the women. Eating nitrates from certain vegetables was linked to increases in thyroid cancer and hypothyroidism, one type of thyroid disease.

This is the first study to show a link between nitrates and thyroid cancer in people, although nitrates have been shown to cause thyroid tumors in animal studies.

Thyroid cancer is the eighth most common cancer among women. In the United States, the incidence of thyroid cancer has increased steadily since 1980.

Nitrate is a common contaminant of drinking water, particularly in agricultural areas where nitrogen fertilizers are used. High rates of fertilizer application may also increase the natural nitrate levels found in certain vegetables, such as lettuce and root crops.

Researchers from the National Institute of Health studied 21,977 older women in Iowa who had used the same water supply for more than 10 years. They determined cancer incidence using the state health registry. They estimated nitrate intake from  public drinking water sources using a public database of nitrate measurements.  Dietary intake was measured through questionnaires. Since nitrate levels in private well water were not available, all private well users were combined into one group.

The results show a nearly three-fold increase in thyroid cancer risk for women with more than five year’s use of a public water supply that had nitrate levels of 5 milligrams per liter (mg/L) or above. The maximum contaminant level of nitrate in drinking water is currently set at 10 mg/L in the United States. There was no evidence of elevated thyroid cancer risk among private well users.