IAEA-SM-224/607 105 have been resected surgically. The incidence of thyroid nodularities and estimated thyroid doses in the various age groups exposed to fall-out are depicted in Table I. It is apparent that more than two-thirds of those individuals in the combined Rongelap and Ailingnae groups, who were under the age of ten at the time of exposure, and over 15% of those exposed over the age of ten have developed thyroid lesions. Four Rongelap children were exposed in utero. One ofthese, a boy exposed at 22—24 weeks gestation (at which time the thyroid was functional), had benign nodules of the thyroid removed at age 20. A much smaller proportion of the Utirik group of either age has developed thyroid abnormalities. The occurrence of three thyroid cancers in the exposed Utirik population (compared with four in the Rongelap group) appears to implicate radiation exposure in the aetiology, but the high incidence is puzzling since it is greater than would be predicted based on Rongelap and Japanese data, and there does not appearto be any increase in benign thyroid tumours in that group compared with the much greater prevalence in the Rongelap group. The high incidence of thyroid nodularity in the irradiated subjectsis in agreement with previous data linking irradiation of the gland with subsequent development of thyroid nodules or carcinoma [6]. Since '3"I is considered much less tumorigenic for thyroid tumours than X-rays, it is rather surprising that, in view of the large contribution of radioiodines to the thyroid dose in the Marshallese, the risk factor (risk/rad) is comparable with that noted following X-ray exposure. This may berelated to the presence of more potentshortlived isotopes of iodine present in fall-out which accounted for two to three times the dose from !74J. Aside from the two subjects with frank hypothyroidism, there has been an increasing suspicion of possible hypothyroidism in other cases. The evidence supporting this conclusion is summarized in Table II. The two boys who developed myxoedema had received an estimated thyroid dose of 1150 rads. In addition at least five of the Rongelap population, who had appropriate testing before surgery, had either hypothyroidism or decreased thyroid reserve [7]. In addition a numberof subjects with sub-total thyroidectomy have shown elevation in serum TSH concentrations and reduction in serum T, whentheir thyroid replacement schedule was not rigorously adhered to. This is significant since in general subtotal, thyroidectomy or lobectomyis not associated with frank hypothyroidism, since the remaining thyroid lobe may often hypertrophy to supply the needed thyroid hormone requirements of the individual. All the subjects so tested and listed in Table II were irradiated at a young age, and therefore received thyroidal dosage of about 800-1150 rads. Because of the suspicion of possible hypothyroidism in individuals to even lower calculated doses, a series of studies of thyroid reserve in previously unoperated exposed Marshallese wasinitiated in 1974, and the following report summarizes the data obtained in this study up to the present time.