42 ectomy in the rat has been reported to cause ade- nomaformation in the remaining thyroidtissue 2 years later.°°*" All these procedures produce hypothyroidism which serves as an effective stimulant to the secretion of TSH by the pituitary gland. The thyroid gland_underthe influence of TSH first undergoes diffuse hyperplasia and hyper- trophy and adenomaformation. In the cases of iodine deficiency and partial thyroidectomy, it is probabie that no drugor carcinogenic agentis in- volved. The incidence of carcinomasafter a com- bination of carcinogenic agents (such as radiation or acetylaminofluorene) and any factor that causes hypothyroidism is much higher.** Radiation by either x rays or '*'I is particularly effective, since it simultaneously acts as a carcinogen and,by im- pairing the functional capacity of the gland, in- duces TSH secretion and thyroid stimulation. The high incidence of adenomatousgoiters and hypothyroidism in the more heavily irradiated children exposed at <10 years of age, compared with no abnormalies in some 100 children in the same age rangein the less exposed and unexposed groups, leaveslittle doubt aboutthe etiological relationship of these lesions with irradiation exposure. The higher incidence in children maybe re- lated to increased sensitivity of the child’s thyroid to irradiation butis probably related more directly to the relative magnitudesof the radiation doses received, the small child’s gland probably having received 5 to 10 times the radiation dose received by the adult’s gland. Since most of these lesions appeared in teen-age children, puberty may have been a contributing factor. The incidenceof the lesions in female childrenis only slightly greater than in male children (1.27 to 1). A greater prevalence of thyroid abnormalities in that sex is correctly noted. The increased incidence of thyroid abnormalities, including one case of malignancy, in the adults of the more heavily exposed Rongelap population makes it necessary to considerseri- ously radiation exposureas the etiological factor. In Appendix 7 statistics on thyroid malignancies in the Marshall Islands and the Trust Territory are presented. A low incidence ofsolitary nodules was noted in older (>50 years of age) unexposed Marshallese of both Rongelap and Utirik Islands. Thesingle nodule case in the less exposed 46-year- old Ailingnae woman, on the other hand, may well fall into the category of the sporadic type, since the nodule was well encapsulated, and the remainderof the thyroid gland appeared normal. Urine analyses indicate that iodine intake is adequate. The incidence of nodules and sporadic cases of goiter seen at the Majuro Hospital* is low, and the Marshallese population showsno evidence for goitrogenic factors in their environment. Thesensitivity of children’s thyroid glands to the development of neoplastic changes from radia- tion exposure has been amply demonstrated. A series of retrospective and prospective studies have clearly shown the causalrelation of irradiation of the neck region in infants and later development of thyroid cancer.°*-** Althoughthe calculation of the dosage in the Marshallese is subject to large uncertainties, the greater incidence of pathological changesin the glands of the Marshallese may be related to a greater dose ofradiation received by their thyroid glands. Lesions similar to those seen in the Marshallese have been reported in children 5 to 11 years old after treatment with '*'T for thyrotoxicosis.**:** Doniach** points out that cell division in the growing thyroid glandof the child may bea factor in the increased sensitivity to irradiation. There appears to be an increased incidence of thyroid carcinoma in inhabitants of Hiroshima and Nagasaki exposed to radiation from the atomic bomb explosions.** These people were exposed to varying doses of external radiation to the thyroid gland but not to internal exposure from radioiodine. It has been assumedthat theslight growth re- tardation previously noted in someof the exposed children was due to radiation, but the mechanism has been obscure. The growth hormonestudies suggested that pituitary function was normal. X rays of the sella turcica showed no evidence of pituitary abnormality. With the recent developmentof hypothyroidism in two of the most growth-. retarded boys and evidence of mild hypothyroidism in several other children with thyroid nodules and growth retardation, a hypothyroid etiology seems mostlikely. Elevated TSH levels indicate that the hypothyroidism is primary. Minimal hypofunction of the gland may have been missed in the past, since the apparently normal PBTlevels may have been spuriously high, the true thyroxine level being masked by the elevated iodoprotein component characteristic of the Marshallese. *in March 1966, twocases of goiter with large, nodular glands were seen at the Majuro Hospital. One may have had mild hyperthyroidism. These cases were not part of the population under study.