Biochemical, hematologic, histometric, structural and ultrastructural changes were studied in the thyroid gland of rats, whom were administered Lithium carbonate. It was noted that (1) the Li carbonate had a cumulative effect on the serum; (2) T3 and T4 hormones in the blood decreased throughout the experiment; (3) the red blood cells, hemoglobin, hematocrit, platelets and leukocytes decreased gradually; (4) the diameter of the thyroid follicles, the size of the follicular cells and colloid droplets decreased, whereas the stroma was invaded by a greater number of collagen fibers and blood capillaries.