Radiation-induced Degenerative Lesions in the Thyroid Gland

Early investigations, both experimental and clinical, created the impression that the thyroid gland is relatively resistant to injury by ionizing radiation (86, 87). The results of the many studies carried out during the past decade, however, not only have reversed this view, but have, in fact, emphasized that destructive lesions of the gland can be readily produced in man and several species of animals by various forms of irradiation. Thus, injury in the human thyroid gland was observed after its irradia tion for thyrotoxicosis, thyroid carcinoma, and cardiac disease. The most recent developments in this field mdi cate: (a) that benign and malignant neoplasms can be made to appear in rat thyroid glands by exposing them to I's' or x-rays (54, 56, 69) ; and (b) that benign nodules and at least one malignant nodule have been found in the thyroid glands of patients long after I― treatment for thyrotoxicosis (75, 76). The administration of even minute doses of I―—l @tc.,for example—has now been shown to have a carcinogenic effect on the thyroid gland of the Long-Evans rat (34), and it is equally significant that thyroid carcinomas have been observed in patients who, during infancy or childhood, were exposed to low dosages of x-radiation to the thyroid area (21). These observations in both animals and man can leave little doubt that irradiation is an important factor in carcino genesis of the gland. In this review we have dealt primarily with three as pects : (a) Histogenesis of the destructive lesions induced in the thyroid gland by various types of internal and external irradiation is discussed. (b) The secondary thyroid epithelial proliferative processes that presumably result from thyrotropic hormone stimulation of portions