Comparative toxicity of /sup 131/I and /sup 132/I in the Wistar rat

The effect of thyroid irradiation from /sup 131/I or x-rays has been extensively investigated. The majority of the results suggest a greater effectiveness per rad for x-rays than /sup 131/I. It has been postulated that dose protraction may be one factor responsible for this difference. To test this hypothesis, Wistar rats were orally administered graded amounts of two beta-emitting radioiodines (/sup 131/I, physical half-life, 8 days; and /sup 312/I, physical half-life, 2.3 hours) and the comparative effects observed for up to 1 1/2 years. The principal parameters evaluated were histopathological changes in the thyroid and blood levels of thyroxine and tri-iodothyronine using radioimmune assays. At two months, the /sup 132/I was about 4X more effective per rad than /sup 131/I in producing a decrease in thyroid weights. Histopathological changes were also greater in the /sup 132/I administered rats. Blood thyroxine was decreased in a dose-related manner in the /sup 132/I treated animals; it was not decreased in the /sup 131/I treated rats. T-3 levels were essentially normal. At 1 1/2 years, surviving rats were sacrificed. The number of thyroid tumors in /sup 132/I treated rats appeared to be greater than in the /sup 131/I rats. The tumors are now being examined histopathologically and quantitative relationships between dose incidence are being evaluated and are reported