apres ryt " ap TorSTEST ee ee —— 4 - . e are es03 TY yore oft reper tarsrw merryi GL le eee Paha arog s peSey sons, the plasma TSH level was 6 uU/mL or greater on two separate the riskof hypothyroidism developing expected number of hypothyroid sub- -shallese in all consistent with the and 4 are of considerable interest in ‘subjects who have not had thyroid jects 25 years after exposure only in iodide I 131 administered deliberate- - those 22 subjects who were younger occasions (Table 4). In all, the plasma than 10 years of age at the time of ly for the treatment of hyperthyroidFT.I was subnormal, though not exposure (mean estimated thyroid extremely low. Four of the six were _ ism.” Such treatment g generally¥ redose, approximately 1,200 rad), only sults in thyroidal doses in excess of evaluated for thyroid gland reserve three such cases. would be expected. by administration of exogenous thy5,000 rad. Based on these studies, it has been estimated that four to five The fact that seven subjects were rotrophin, and all had a subnormal already hypothyroid within 4 years incrementin plasmaT,,. Infive, proti- cases of primary hypothyroidism per of exposure indicates that the preva10° persons per rad per year would relin infusion tests were performed. lence of this complication is considerThe plasma TSH level at 20 minutes appear from thyroidal exposure to "I was in excess of the upper limits of greater than 2,500 rad.” ably higher than predicted. In the normal observed in euthyroid Mar. The results presented in Tables 3 surviving Rongelap and Ailingnae modest elevation in basal TSH levels. Before surgery for removalof a thyroid nodule in 1976, one other subject with a calculated thyroid dose of 425 rad had basal plasma TSH concentra- tions of 5.1 and 4.9 xU/mL, and the plasma TSH level after protirelin administration was 36 »U/mL at 20 minutes and 48 n.U/mL at 40 minutes, after various quantities of sodium Hight of the aforementioned estimate. Nineof the subjects listed in Tables 3 and 4 who had moderate to severe thyroid hypofunction were all young at the time of radiation exposure and, therefore, received the highest dose of radiation a5 a consequence of the -small size of the thyroid gland. The thyroid hypofunction in the older suggesting early thyroid dysfunction. . subjects with lower exposure (Table The increment in plasma T, concen4) was less pronounced, but the plastration 24 hours after thyrotrophin _ ma TSH concentrations are consistadministration was 0.8 yg/dL, and ently greater than those found in the the FT,I was 5.6 units. contro] Marshallese population, and Because most of the subjects in Table 4 were between the ages of 50 and 69 years, evaluation of plas- - ma TSH levels in 53 unexposed Marshallese between the ages of 50 and 70 years was performed. In only one person, subject No. 982, mentioned | previously, was the TSH level greater than or equal to 6 pU/mL. Ninety percent of the plasma TSH concentra- tions were less than 3 ypU/mL in the older group, as in the entire Mar- shallese control population sample. Titers of antimicrosomal and antithy- roglobulin antibodies were normal in plasma from exposed subjects. COMMENT | The association of radiation exposure to the thyroid gland with the subsequent development of thyroid nodules is well recognized.*"* The appearance of hypothyroidism after accidental the same TSH assay as employed in these studies, a plasma TSH level of 6 zU/mL maintained for six hours by protirelin infusion results in substantial thyroid gland stimulation in normal. persons." Further evidence of decreased. thyroid reserve in the exposed Marshallese is the decreased response to thyrotrophin and enhanced protirelin responsiveness. These abnormal plasma TSH concen- trations are not found in a compara- bly aged, euthyroid, unexposed population, indicating that this is not a manifestation of age alone. No indi- vidual in-either group had elevated titers of antithyroid microsome or antithyroglobulin antibodies. The estimated dose to the thyroid in the subjects'in Table 4 was about one third of that of the subjects listed in Table 3, with the exception of patient exposure to radioactive - No. 32. Virtually all of the persons found to iodines contained in fallout has not been reported previously except in this Marshallese population. Studies by Rallison et al did not show an increased number of cases of overt hypothyroidism in children exposed to low levels of fallout in Utah. Most previous studies of the effects of radioiodine on human thyroid function have consisted of evaluations of 1574 the FTIvalues are all reduced. Using have evidence of thyroid hypofunction were on Rongelap at the time of fallout exposure. None of the Utirik population and only one of those on Ailingnae (No. 16) was affected. If the risk of hypothyroidism associated with '"I therapy for hyperthyroidism (four to five cases per 10° persons per rad per year) is used to calculate the JAMA, March 19, 1982—Vo! 247, No. 11 surgery (n=36), only two cases of hypothyroidism would be expected, assuming an average dose of 400 rad. Five such persons were observed. The major reason for this apparent discrepancy mayderive from the type of radiation received by the Mar- shallese. Perhaps only one third of the thyroid dose in the Rongelap and Ailingnae subjects was due to '"I, the remainder being derived from the -shorter-lived isotopes "I, "I, and I. There are reasonably good data -to suggest that radiation from '"I is only one fourth to one tenth as effective as an equivalent dose of x-irradiation in producing thyroid damage owing to the higher dose rate achieved with the latter.’*” However, the short-lived isotope '"I is as effective in destroying thyroid tissue as is x-irradiation, and the same may be true of '"I and “7.9 Hence, the use of figures from data on thyroid radiation effects due to '"Y will give a considerably lower ‘estimate for hypothyroidism than would be expected for the Marshallese subjects, owing to the significant contribution from short-lived, more ‘biologically effective isotopes of iodine. . The following calculation shows the potential importance of the shortlived isotopes of iodine to the estimates of thyroidal dose. It is given as an example only. and should not be used as a revised dose calculation for this population. If the total dose to the thyroid due to internal deposition of radioiodines were three times that due to J and "I, I and '’I are assumed to be roughly seven times (range, four to ten times) more destructive per rad than '"I,?~ a ‘""'T equivalent” dose to the thyroid can be derived. For example, a person receiving a total dose to the thyroid of 900 rad would sustain 300 rad from "I, Hypothyroidism and Fallout Exposure—Larsen et al 5017588 ate Ae al ee aa reanererears tee ge te oe on jeRe hg be er] Saisiebora aNeeoeereeeone poate z