Treatment of Hyperthyroidism 87 Race. In two series race is taken into account. Apparently blacks are more resistant to therapy, requiring multiple doses more often than whites.’ In addition, there is less spontaneous, idiopathic, and post-therapeutic (*'I and surgery) myxedemain black patients.'* Previous surgery. In all series the use of '*'I following a partial thyroidec- tomy resulted in a greatly increased incidence of hypothyroidism.”* Size of postsurgical remnant. The size of the postsurgical remnant has no correlation with the incidence of subsequent hypothyroidism.''° Immune status. The incidence of post-therapy hypothyroidism has been considered in relationship to a number of immunefactors. In the surgical series of Green and Wilson,’ there appeared to be a higher incidence of hypothyroidism in patients whose surgical specimens showed evidence of focal thyroiditis. A slight correlation existed between focal thyroiditis and thetiter in the tanned red cell agglutination and complementfixation tests. No consistent effect of operation on the titers was found. Others have also noted the association of thyroiditis in the surgical specimens with postsurgical hypothyroidism.*!%?! On the other hand, in '*'I-treated patients, there was no correlation between the results of serologic tests and the clinical state following treatment. In addition, the 1°] therapy had no effect on titer levels in patients in whom this was tested prior to and 2 to 3 years after treatment.* The results of other studies are not clear. Burke and Silverstein?? demonstrated a relationship between increased levels of circulating antithyroid antibodies and '*‘I-induced hypothroidism in the first year. Others have also found transitory increases in thyroid autoantibodies in the first year following radioiodine therapy for hyperthyroidism, greater in those becoming hypothyroid in the first year, but probably not significant for the occurrence of late hypothyroidism.??74 A lack of significant relationship of humoral antibodies to development of post-therapeutic hypothyroidism following ''I has been found by others.7:25-26 There does appear to be a higher incidence of postsurgical hypothyroidism in patients who have hightiters of antithyroid antibodies.?>?¢ Relationship to prior nondestructive therapy. Segal et al. found the lowest incidence of post-'*!I hypothyroidism in patients who had received iodine or antithyroid drugs and iodine prior to !*11. The incidence in this group was 4.1 percent compared to 9.4 percent in those having no prior treatment and 23.6 percent in those who had prior drugs and destructive therapy such as x-ray and/or surgery. ETIOLOGY OF POST-THERAPEUTIC HYPOTHYROIDISM ey J Px fi seen CF} The reason for the high incidence of post-therapeutic hypothyroidism following 7'I is still unknown, but a number of hypotheses have been proposed. Although radiation effects may be a major cause, it does not explain why similar