The effects of taking chronic nitrate by drinking water on thyroid functions and morphology

Nitrate incorporation in humans takes place via drinking water and food. The water used for drinking and cooking in the goitrous areas is high in nitrate content. The aim of the present study was to evaluate both chronic effects and the dose-response relationship of nitrate on thyroid functions. A total of rats were divided into 5 work groups and sodium nitrate was added to their drinking water in different concentrations (0, 50, 100, 250 and 500 mg/l) over a 30-week period. The radioiodine uptake of thyroid was decreased in the 50 mg/l nitrate group, whereas it was increased in the 250 and 500 mg/l nitrate groups as compared to control. All hormones of thyroid gland except total thyroxine and thyroid-stimulating hormone were decreased in the 50, 250 and 500 mg/l nitrate groups. However, the level of total thyroxin was increased in the 100 mg/l nitrate group. Thyroid gland weights were increased in all experimental groups. Histomorphological changes were observed in the 250 and 500 mg/l nitrate groups. These findings suggest that nitrate impairs thyroid function involving the hypothalamo-hypophysio-thyroid axis. This observation could contribute to the current discussion about the acceptable daily intake of nitrate, as well as drinking water nitrate standard safety margins.