3. The Developmentof Thyroid Abnormalities Fram In Utero Exposure The developmentof thyroid adenomas in one of the Marshallese boys exposed in utero is of interest. Four children were exposed in utero: in the higher radiation group,one at the end of thesecond trimester and twoat the end ofthe first trimester; in the lower dose Rongelap group,one at the end of the second trimester. Only the boy in the higher dose group and exposed in the second trimester has developed thyroid lesions. Several cases have been reported of children who had myxedema following in utero exposure during treatment of the mother with large doses of radioiodine.138-141 However, this Marshallese boy appears to be the first reported case of thyroid adenomas developing presumably from zz utero exposure to radioiodines. In utera thyroid dose calculations are not possible, since insufficient data are available on fetal thyroid uptake of radioiodines from the motherat various stages of gestation.!42 Figure 48 shows thyroid function of the humanfetus. The glandis not thought to begin to function and to accumulate iodine until about the 12th week of gestation.144.145 At the time of exposure of the Marshallese boy (about 22 to 24 weeks) the thyroid should have been actively functioning. He received 175 rads of gamma radiation but the dose from radioiodines is uncertain. In view of the high uptake of radioiodines per gram bythefetal thyroid at 22 weeks, the thyroid dose might be expected to be high. However, since it appears from our data that 40.0 + ] 30.0 J 23.0' 4 200} J 150+ j 10. 0.05L Bus 4 genic effect in rats and dogs is equal on a rad basis to that of x rays. Walinder et al. have reported similar findings.!54-155 Dunning,}36 in estimating thyroid dose from absorptian of radioiodines in fallout, considers the shorter-lived isotopes to be 4 times as energetic as 141], The more energetic beta radiation with a higher dose rate and longer range gives a more uniform tissue distribution of radiation than is produced by 1!3![. This might explain why the incidenceof thyroid lesions in the children receiving 1000 radsfalls on a line corresponding to 1000 rads of x-radiation (Figure 44), or it might indicate that the radioiodine exposure was about 10 times as effective as 1511 alone would have been (see Cole'37), Thus the two boys with atrophy of the thyroid may have had doses to the gland comparable with 4000 to 6000 rads or more of xirradiation. 50 ee G9 1015 20025 Fetal age, in weeks 30 35 40 | Child | At birth a ——~ ee 62 Adult Infant Figure 48. Thyroidal uptake of 1311 (% per gram) versus age. (From Evans et al.!43) longer latent periods are associated with lower doses of radiation to the thyroid, it is likely that the thyroid in this boy exposedin utero received a lower dose than that in Marshallese children exposed at a youngage.Also, this boy and the others exposed in ulero have shown no impairment in growth and development. It is of interest that the motherof this boy has not developed any thyroid abnormalities and appears to be euthyroid. The two children exposed at the end ofthe first trimester probably had nonfunctioning primordialthyroid glands at that time so that the glands received only the gamma exposure and weretherefore less likely to develop thyroid abnormalities. None of the four children exposed in utero has shown microcephaly or mental retardation, which have been noted in some of the children exposed in utero in Japan. V. Neoplasia Amongthelate effects of radiation, the enhancement of the development of neoplasia has been well documented. In irradiated animals, malignancies not only may be induced at anearlier age but appear in increased numbers. In the Japanese atom bomb survivors, leukemia and thyroid neoplasia have shownthe most clear-cut association with radiation exposure, though morerecently