Increased nitrate intake might affect the thyroid gland function in humans, as has been observed in animals. The reason is that the nitrate-ion (NO3-) inhibits the iodide (I-) transport into the thyroid gland because it shares the same transport mechanism. This inhibition could lead to a decrease in thyroid hormone (T4, T3) secretion, followed by an increase in the thyroid-stimulating hormone (TSH). In the end thyroid gland enlargement (goitre) could occur. Since there is only weak epidemiological evidence that thyroid gland enlargement may occur in humans, our aim was to investigate the effect of nitrate on the human thyroid gland function by means of a four-week nitrate exposure study. Once a day for 28 days ten volunteers received an oral solution of 15 mg sodium nitrate per kg body weight (three times the allowed daily intake, ADI) in 200 ml distilled water (nitrate group), and ten volunteers received 200 ml distilled water (control group). Both groups followed an iodine-restricted and low-nitrate diet; this was checked by measuring urinary iodide and plasma nitrate concentration. Before and after the 28-day exposure period the percentage (%) radioiodine (131I) uptake (RAIU) was measured 5 hours and 24 hours after 131I-capsule intake to investigate the competition of nitrate in the iodide transport. Before (nitrate) exposure and 2, 3 and 4 weeks after the start of the exposure period blood samples were taken to measure the hormones T4, T3, rT3, TSH and IGF I to investigate the thyroid gland function. Nitrate was found to have no effect on the hormone concentrations during the four-week nitrate exposure of three times the ADI of nitrate. Within the nitrate group the 24-hr RAIU had increased 1.5 times (from 20 % to 29 %) after 28 days of nitrate exposure compared to the 24-hr RAIU before exposure, while a decrease in 24-hr RAIU in the nitrate group had been expected. So we can conclude that an exposure of three times the ADI of nitrate will not cause changes in thyroid gland function in a healthy population.