Leukemia, Radiation, and Hyperthyroidism

Saenger et al. (Letters, 19 Mar.) argue that hyperthyroidism per se is responsible for the statistically significant increase in leukemia death rate which they observed in a combined group of 18,379 radioiodine-treated (RAI) and 10,731 surgery-treated (SUR) hyperthyroid patients [1]. For the years 1946–1964 this increase amounted to 1.5 times the expected death rate calculated on the basis of the experience of the U.S. general population. I have examined the origins of this increase by comparing observed numbers of leukemia deaths in each patient group with the corresponding expected numbers calculated on the basis of United States age-, sex-, year-specific leukemia death rates [2]. The results show that the excessive leukemia death rate is largely confined (i) to patients who were over 50 years at the time of treatment; (ii) to patients with the acute rather than the chronic form of the disease; and (iii) to the RAI rather than the SUR group. Moreover, in RAI patients there is a statistically significant excess of acute leukemia deaths [2] occurring between ages 50 and 79 (namely, 9 observed versus 3.6 expected; P = .01); whereas, in SUR patients, although there is an excess of such deaths between ages 50 and 79 (4 observed versus 2.5 expected), the excess is not statistically significant [3].