Incidence of Leukemia Following Treatment of Hyperthyroidism: Preliminary Report of the Cooperative Thyrotoxicosis Therapy Follow-Up Study

This study was conducted to test for an increase in the incidence of leukemia in hyperthyroid patients treated by radioactive iodine 131 I as compared to the incidence in patients treated differently. Of 36,000 patients from 26 medical centers, approximately 22,000 were treated with 131 I and about 14,000 were treated surgically or solely with antithyroid drugs. More than 96% of patients were followed up. Forty-four cases of leukemia occurred after, and 16 before, treatment. The incidence of leukemia in patients treated with 131 I for hyperthyroidism and the incidence in those treated surgically do not differ. Patients both undergoing surgery and receiving 131 I have a higher incidence of leukemia than any other treatment group. The observed mortality from leukemia for hyperthyroid patients is 50% higher than that of the US population.