Thyroid neoplasms in the rat: a comparison of naturally occurring and I131-induced tumors.

The widespread use of I131in the diagnosis and treatment of thyroid disease in man has focused attention on the effects of this radioactive isotope in experimental animals. In 1951, Goldberg and Chaikoff described the development of benign and malignant thyroid tumors in Long-Evans rats 1.52 years after the animals had received single in jections of 400 tic. I131(7, 8). Doniach also found benign and malignant thyroid neoplasms in rats treated with I131alone or in combination with methyl thiouracil and acetylaminofluorene (3, 4). In a recent report (6), on the other hand, neo plasms were not observed in rats that had been given injections of I131. An extensive study of benign and malignant thyroid tumors induced, in the rat, with radioiodine is presented here. This report is based on examination of 198 Long-Evans rats, 1.5-3 years following a single injection of I131.One hundred fifty-six rats of the same age served as controls. The production of malignant thyroid tumors in the Long-Evans rat by means of I131injections is con firmed. A high incidence of spontaneous carcinoma of the thyroid gland was noted, but these naturally occurring lesions are cytologically and histologically unlike the malignant lesions induced by I"1.