The effect of increasing levels of sodium nitrate on urinary iodine excretion, changes in the morphology of thyroid follicles, and thyroid gland hormonogenesis were studied. In addition, effects of nitrates on serum lipoproteins were examined. Twenty-four, 5-week old, growing male rats of Wistar strain were randomly assigned to four experimental groups (I – 0, II – 500, III – 1500, and IV – 3000 mg NaNO₃ per kg body weight). For 3 weeks the rats were fed restricted amounts of AIN’93G diets and had free access to distilled water. Body weight of the animals was recorded weekly. No external signs of dietary nitrate toxicity were observed in this study. However, urinary iodine concentrations tended to decrease with increasing nitrate intakes (13.19 μg/dL in Group I vs. 7.55 μg/dL in Group IV). The nitrate-fed rats showed histological changes in thyroid follicles. The thyroids were diffusely hyperplastic with small follicles. The epithelial follicle cells were also significantly higher in the nitrate-fed rats. In addition, mild to moderate irregularity of hypertrophic follicular cells and decreased amount of colloid were observed in the nitrate-fed animals (Groups II, III, and IV). Finally, we found that the vascularity of the thyroid tissue from nitrate-fed rats was much more developed, compared with the control animals. Serum fT₄ levels (pmol/L), tended to be decreased in the nitrate-fed rats. On the other hand, the dietary nitrate doses of 1500 and 3000 mg/kg body weight, caused highly significant increases (65% and 300%, respectively) in circulating levels of serum TSH (p<0.001). Feeding incremental doses of nitrate to rats did not result in significant increases in serum lipoproteins (total cholesterol and LDL-cholesterol and HDL-cholesterol). In contrast, the highest dietary nitrate level increased significantly (p<0.05) serum triacylglycerol concentrations by 45.5%, compared to the control group of rats. Therefore, nitrate may be considered as a competitive iodine inhibitor, affecting the thyroid-pituitary hormonal axis, in a way similar to that of iodine deficiency, thus acting as a goitrogen.