HISTOPATHOLOGICAL CHANGES INDUCED IN THE NORMAL THYROID AND OTHER TISSUES OF THE RAT BY INTERNAL RADIATION WITH VARIOUS DOSES OF RADIOACTIVE IODINE1

Abstract

Rats of the Long-Evans strain, weighing about 250 g., were given single, intraperit. injns. of 18, 300, 525, or 875 microcuries of I13l. Autopsies were performed on groups at short intervals up to 3 days, then weekly up to one month, and thereafter at 5, 6, and 8 mos. after the injn. No evidence of damage to any tissue was observed at any of the intervals after injecting 18 microcuries of I131. a progressive degenerative process was observed in the thyroid glands of animals that received 300 or 525 microcuries of I131. This was arbitrarily divided into 5 stages (1) degeneration and necrosis of epithelial cells with follicular disruption; (2) vascular degeneration and thrombosis; (3) acute and chronic inflammatory changes; (4) fibrous organization; and (5) epithelial regeneration. The thyroids of rats that had been injd. with 875 microcuries responded in a similar fashion except that no epithelial regeneration occurred. No thyroid epithelial elements were found in rats 8 months after a single injn. of 875 micrbcuries of I131. Cytological changes were detected in thyroid glands as early as 12 hrs. after single injns. of 525 or 875 microcuries of I131, and as early as 24 hrs. after 300 microcuries. Regeneration to normal thyroid structure was found in animals which had been injd. with 300 microcuries of I131. Five to 8 months after the injn. of 525 microcuries of I131 atypical cells, single and in groups, were observed. These cells showed some resemblance to the so-called Hurthle cells found in certain thyroid lesions in man. Parathyroid glands were found in all animals that had been injd. with I131, and these glands appeared viable. Some fibrosis had occurred in the peripheral portions of the glands, particularly when 525 and 875 microcuries of I131 were injd. Evidence of tracheal damage, and slight reversible changes in the kidneys, were noted. No pathological effects were observed in the livers of these animals.