metabolic functioa of the thyroids of young children (237). it was observed, however, that the risk for carcinoma was greater in the older exposed groups than in the children. (See Table 4, Appendix IV.) This may be related to the higher doses in the children (“overkill effect"). (See Appendix II.) The finding of thyroid nodules in two children who were exposed in utero on Rongelap indicates the importance of fetal irradiation of the thyroid gland by radioiodines absorbed by the mother. exposed in utero on Rongelap, Since there were only three children this finding seems significant. Though thyroidal hypofunct on had been noted earlier in some exposed Rongelap children, this finding has been more recently noted in adults of this population who had received lower thyroid doses and who had not exhibited thy~roid nodularity. The appearance of hypothyroidism following accidental expo- sure to radioactive iodines contained in fallout has not been previously re- ported except in this Marshallese population. Studies of Rallison et al. did not show an increased number of cases of overt hypothyroidism in children exposed to low levels of Fallout in Utah (181). Most previous studies of the effects of radioiodine on human thyroid function have consisted of evaluations of the risk of hypothyroidism developing after various quantities of !3ly administered for the treatment of hyperthyroidism (173,209). Such treatment generally results in thyroidal doses >5000 rads. The most severe thyroidal hypofunctio: was noted in 6 of 22 Rongelap children exposed at age <10 years (Table 8). As pointed out, this group had received thyroid doses about three times as great as that of the adults because of the smaller size of their thyroid glands. While the thyroidal hypofunction in the prospectively studied, older-age group (Table 7), was generally less pronounced, the plasma TSH concentrations were considerably greater than those found in the control Marshallese population and the FT,I results were reduced. It is pertinent to note that, using the same TSH assay as em- ployed in these studies, a plasma TSH of 6 W/ml maintained for 6 hr by TRH infusion results in significant stimulation of the thyroid glands in normal individuals (132). Further evidence of decreased thyroid reserve in this prospective study was a decreased response to TSH and enhanced TRH responsiveness. Thyroidal hypofunction vas noted in only one individual exposed on Ailingnae, none in the groups exposed on Utirik Atoll, and only one in the unexposed group. The abnormal plasma TSH concentrations were not found in a comparably aged, euthyroid, unexposed population, which indicated that this finding was not a manifestation of age alone. No individuals were found to kave elevated titres of antimicrosomal or antithyroglobulin antibodies. On the basis of studies of !3!r trearment of primary hypothyroidism, it has been estimated that 4 to 5 cases of hygothyroid:sm/ 10 persons/year/rad would appear from thyroidal exposures to lt in excess of 2500 rem (163, 173,174). This riak factor was used to estimate the number of hypothyroid cases that might be expected in the more heavily exposed Rongelap population at 25 years after exposure. In the 22 Rongelap individuals vho were <10 years of age at the time of exposure (using a mean estimated dose of 1200 rads), only 3 hypothyroid cases would be expected, whereas 7 of these children were hypothyroid within 14 years of exposure. Among 45 Rongelap individuals who were >10 years of age at the time of exposure (average thyroidal dose of about 387 rads), two cases of hypothyroidism would be expected whereas four were noted in this group. These numbers are considerably higher than expected on the basis of !31] risk data. - 79 -