© € Radiation-Associated Thyroid Carcinoma 243 THYROID DOSIMETRY The fallout produced several possible sources of radiation exposure to the thyroid gland in addition to the gamma radiation. Iodine isotopes are produced in relatively high yields by the fission process. Some are too short-lived to be of consequence, but iodine 131, 132, 133, and 135 are sufficiently long-lived to have been present at the time the fallout cloud arrived at the inhabited atolls. Thyroid lesions were not expected, on the basis of earlier estimates of thyroid dose from internal absorption of radioicdines. In 1964 when thyroid nodules first began to appear, thyroid doses were re-evaluated by James (7). Calculations of the thyroid dosage from radioiodines unfortunately had to be based on the 13l1I content of a single pooled urine sample from Rongelap people collected at 15 days post exposure. The shorter-lived isotopes of iodine delivered 2 to 3 times the estimated dose delivered by 131I alone. The importance of these shorter-lived, More energetic isotopes must be kept in mind in assessing the dose-response relationship for thyroid effects. The dose to the thyroid of a Rongelap adult, including gamma dose, was calculated to be about 335 rads (220450 rads) and to that of a 3-year-old Rongelap child as 700-1400 rads. The larger doses in the children were due mainly to the smaller sizes of their thyroids. The thyroid doses in the lesser exposed populations were extrapolated from those of the Rongelap group on the basis of the ratio of gamma doses. Individual children's doses were calculated by using a linear relationship between the estimated thyroid size and regression lines drawn for the three exposed populations. EARLY THYROID EFFECTS Beginning several years after exposure it was noted that 5 of 19 children exposed at < 10 years of age iD Pm Lad c= CFI showed retardation of growth. Examinations during this early period did not reveal any recognizable thyroid abnormalities, and the protein bound iodine (PBI) levels in these children as well as in all Marshallese were in the normal to high normal range. Two boys