Iodine content of serum thyroglobulin in normal individuals and patients with thyroid tumors.

We indirectly estimated the iodine content of serum thyroglobulin (TG) in normal individuals and patients with benign and malignant thyroid tumors. Because insufficient TG is present in the serum to perform chemical determinations, equilibrium density centrifugation was used to determine its density, a measure of TG iodine content. In five patients undergoing thyroidectomy, serum TG was compared to TG extracted from the nodules and TG from the surrounding normal thyroid tissue. The iodine content of the tumor TG was much less than that of normal TG in four of the five patients. In patients with benign and malignant nodules, the iodine content of serum TG was lower than that of normal TG, and it was similar in patients with benign and malignant disease. In normal individuals, serum TG was also poor in iodine, similar to the serum TG from the patients, and in the same position as TG with virtually no iodine. These findings are in accord with our report that serum TG in rats is nearly completely devoid of iodine. TG could enter the circulation either by secretion of newly synthesized TG or release of stored TG from the thyroid. The findings show that serum TG in normal individuals does not result from the release of preexisting TG. More likely, it arises from the secretion of poorly iodinated, newly synthesized molecules. Since the elevated serum TG found in patients with nodules also is poor in iodine, it must come directly from the tumor rather than from destruction of surrounding normal thyroid tissue.