53 who had been exposed at age 10 to 20 vears, gave results similar to those obtained in 1965. Thus the Rongelapese do not showa trend toward decreasing thyroid iodine uptake as do North Americans (attributed to increased dietary iodine intake).9° The main purposeofthe later study, however, was to see whether the exposed individuals in the intermediate age group might be developing thyroid failure despite the absence of thyroid experience of many of the thyroidologists involved in this study thatit is extremely difficult to make a clinical diagnosis of hypothyroidism in this population. This difficulty emphasizes the importance of the plasma TSH measurement, which ts now recognized as the-most sensitive indicatorofprimarythyroid dysfunction. The status of thyroid gelap people without thyroid abnormalities are now showing evidence of reduced function on the basis of response to TSH stimulation.) Radioimmunoassays (RIA) for T, and TSH have been carried out since 1972 on the exposed Rongelap group and on other people who have had thyroid surgery. The results on subjects with knownthyroid lesions are presented in Table 27. OF the 32 subjects tested, 17 or 33% had atleast one TSH level above the upper limits of normal (these are in boldface in Table 27). These findings indicate that the residual thyroid tissue is inadequate to sustain euthyroidism in these cases and also reflect inadequate adherence to the prescribed T4 replacement regimen. Inadequate T, replacement ts apparently a chronic problem for 3. Thyroid Status of Exposed Rongelap People Without Apparent Thyroid Lesions nodules; its results indicated no evidence forthis. (However, as discussed below, some exposed Ron- certain patients (Nos. 5, 23, 33, 65, and 72). Ele- vations of plasma TSH have been observed also in a number of exposed people without known thyroid lesions (the TSH concentration was >10 pU/ml only in subjects No. 71 and 74 - see Table 28 and Appendix 8). Presumably these subjects, as well as several others with plasma TSH concentration >5 but <10 pU/ml (Nos. 4, 16, 34, 47, 68, and 78), are not receiving the T, therapy 2s regularly as had been hoped. In the Rongelap control group plasma T,4 concentrations were deterniined in 109 subjects. In those with T, <5 pg/ dl (6% of those tested) TSH was determined; no elevated levels were found (data not shown). Only 1 of 99 Utirik subjects tested has had an elevated serum TSH level (No. 2232). These normal findings in the unexposed and Utirik groups suggest that in the irradiated Rongelap group there is impaired thyroid function without palpable lesions which could become symptomatic in the future. The test results probably lead to underestimation of the true incidence of impaired thyroid function, since presumably many of the patients are taking the medication as directed. It should be noted that it is the personal function in exposed people without apparent thy- roid lesions was further tested with exogenous TSH in 1974, as described below. In preparation for TSH testing of reserve thy- roid function, prophylactic Ty medication was dis- continued for 2 months before the 1974 survey in all exposed subjects without recBgnized lesions. During the survey, pla%ma samples weeg.obtained before and 24 hrafter intramusculariinjection of 10 units of bovine TSH (Thyrotropar, Armiéur). Both sets were analyzed for T, and the first set ' also for TSH. Theresults, and thyroxine-binding globulin-binding capacities (T BG-binding capacities) in somecases, are given in Table 28. The mean incrementin plasma T4 following TSH was 2.354 1.2 pg/di (mean + S.D.). The mean Ty prior to TSH injection was 6.61.7 pg/dl. Similar tests88 on 13 subjects at the University of Pittsburgh showed a mean incrementin plasma-T.40f 4.7> 1.0 pg/dl, and a baseline plasma:‘Ts of7.3 pg/dl whichis not significantly different from atofthe exposed Rongelap group being tested. Thus, the T4 response to TSHissignificantly less (p<(0.001) in this group of 26 exposed Rongelap subjects than in the group of 13 subjects from theJnited States. Because of the possibility that the smaller incre- ment in plasma T4 24 hr after TSH in the exposed subjects was due to factors other than decreased thyroidal reserve, TSH stimulation tests were done on 10 euthyroid unexposed Rongelap and Utirik people during a subsequent stfrvey. The mean initial plasma T4 in thig group was 6.01.7 pg/dl; and the meanincrenfeént 24 hr after TSH injection was 4.2 1.3 pe/dl, significantly greater (p<.0.001) than in the exposed subjects. These results and the finding ofelevated plasma TSHlevels suggest that there is underlying,clinically inapparent, thyroid damagein the exposed Rongelap population. Whileit is conceivable that the T, replacement program mayhaveled to decreased thyroid reserve, the test results indicate a need for continuedclose follow-up of the exposed