96 Harold L. Atkins The study was discontinued after 36 months follow-up because ofthe high rate of persistent hyperthyroidism (58 percent) along with an incidence of 9.7 percent hypothyroidism. This compares with 32 percent hyperthyroidism and 10.7 percent hypothyroidism following '7"1. Forfull evaluation of !*I more time must pass and thelate results at 15 to 20 years be apparent. Hypothyroidism following !5I may be moretransient than after 1317, Assessmentof !*] at this time does not appear to be particularly encouraging with regard to its ability to restore the euthyroid state without excessive hypothyroidism. CONCLUSIONS Hypothyroidism occurring within the first year of radioiodine therapy can be reduced by lowering the amountof administered 14]. The incidence appears to be directly related to the dose in «Ci/g.” This probably reflects direct killing of thyroid cells or a mechanism related to interphase death. Late-onset hypothyroidism does not appear to be directly related to the administered dose of radioiodine. It is probably a result of cell death during mitosis and mayberelated to biological factors affecting the rate of cell replication. This late-onset hypothyroidism remains a problem. The various methodsthat have been applied to reduce the incidence have had unimpressive results. The wide variation in results from one locality to another suggests that environmental and dietary factors may also be important. The differences between surgical and radioiodine therapy results may be due largely to a bias in selection of patients. If so, improved results with less posttherapeutic hypothyroidism must await a better understanding of the etiology of thyrotoxicosis, and treatment must be directed toward that etiology. REFERENCES 1. Chapman, E. M., Maloof, F.: The use of radioactive iodine in the diagnosis and treatment of hyperthyroidism: ten years’ experience. Medicine (Baltimore) 34:261321, 1955. 2. Beling, V., Einhorn, J.: Incidence of hypothyroidism and recurrence following '*"I treatment of hyperthyroidism. Acta Radiol. 56:275-288, 1961. 3. Segal, R. L., Silver, S., Yohalem, S. B., et al.: Myxedemafollowing radioactive iodine therapy of hyperthyroidism. Am. J. Med. 31:354-364, 1961. 4. Green, M., Wilson, G. M.: Thyrotoxicosis treated by surgery or iodine-131. With special reference to development of hypothyroidism. Br. Med. J. 1:1005-1010, 1964. 5. McGirr, E. M., Thomson, J. A., Murray, I. P. C.: Radioiodine therapy in thyrotoxicosis. A review of 908 cases. Scot. Med. J. 9:505-513, 1964. 6. Dunn, J. T., Chapman, E. M.: Rising incidence of hypothyroidism after radioactive- iodine therapy in thyrotoxicosis. N. Engl. J. Med. 271:1037-1042, 1964. 7. Nofal, M. M., Beierwaltes, W. H., Patno, M. E.: Treatment of hyperthyroidism with sodium iodide I 131. A 16 year experience. J.A.M.A. 197:605-610, 1966. 8. Blahd, W. H., Hays, M. T.: Graves’ disease in the male. A review of 241 cases treated with an individually calculated dose of sodium iodide I 131. Arch. Int. Med. 129:33-40, 1972.