33 whohad been exposed, at age 10 to 20 vears, gave results similar to those obtained in 1965. Thus the Rongelapese do not showa trend toward de- creasing thyroid iodine uptake as do North Americans (attributed to increased dietary iodine intake). The main purpose of the later study, however, was to see whether the exposed individuals experience of manyof the thyroidologists invoived in this study thatit is extremely difficult to make a clinical diagnosis of hypothvroidism in this pop- ulation. This difficulty emphasizes the importance of the plasma TSH measurement, which is now recognized as the most sensitive indicator of primary thyroid dysfunction. The status of thyroid in the intermediate age group might be developing thyroid failure despite the absenceof thyroid function in exposed people without apparentthyroid lesions was further tested with exogenous TSH gelap people without thyroid abnormalities are now showingevidence of reduced function on the basis of response to TSH stimulation.) 3. Thyroid Status of Exposed Rongelap People Without Apparent Thyroid Lesions nodules; its results indicated no evidenceforthis. (However,as discussed below, some exposed Ron- Radioimmunoassays (RIA) for Ty and TSH have been carried out since 1972 on the exposed Rongelap group and on other people who have had thyroid surgerv. The results on subjects with knownthyroid lesions are presented in Table 27. Of the 32 subjects tested, [7 or 53% had at least one TSH level above the upper limits of normal (these are in boidface in Table 27). These findings indicate that the residual thyroid tissue is inadequateto sustain euthyroidism in these cases and also reflect inadequate adherence to the prescribed T, replacement regimen. Inadequate T, replacementis apparently a chronic problem for certain patients (Nos. 5, 23, 33, 65, and 72). Ele- vations of plasma TSH havebeen observed also in a numberof exposed people without known thyroid lesions (the TSH concentration was > 10 uU/mi only in subjects No. 71 and 74 - see Table 28 and Appendix 8). Presumably these subjects, as weil as several others with plasma TSH concentration >5 but <10nU/ml (Nos. 4, 16, 34, 47, 68, and 78), are not receiving the T, therapy as regularly as had been hoped. In the Rongelap control group plasma T4 concentrations were determined 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 Ron- gelap group thereis impaired thyroid function without palpable lesions which could become symptomatic in the future. Thetest results probably lead to underestimation of the true incidence of impaired thyroid function, since presumably manyofthe patients are taking the medication as on CH CD co directedNggould be noted that itis the personal in 1974, as described below. In preparation for TSH testing of reserve thyroid function, prophylactic T, medication was dis- continued for 2 months before the 1974 survey in all exposed subjects without recognized lesions. During the survey, plasma sampies were obtained before and 2¢4 hr after intramuscularinjection of 10 units of bovine TSH (Thyrotropar, Armour). Both sets were analyzed for T4 and the first set also for TSH. The results, and thyroxine-binding globulin-binding capacities (TBG-binding capaci- ties) in somecases, are given in Table 28. The mean increment in plasma T, following TSH was 2.35 1.2 ug/dl (mean + S.D.). The mean T, prior to TSH injection was 6.64 1.7 pg/dl. Similar tests§8 on 13 subjects at the University of Pittsburgh showed a mean incrementin plasma Ty, of 4.7 1.0 pe/dl, and a baseline plasma T, of 7.3 ug/dl whichis notsignificantly different from that ofthe exposed Rongelap group being tested. Thus, the T4 response to TSHis significantly less (p<0.001) in this group of 26 exposed Rongelap subjects than in the group of 13 subjects from the United States. Becauseofthe possibility that the smaller incrementin plasma T, 24 hr after TSH in the exposed subjects was dueto factors other than decreased thyroidal reserve, TSH stimulation tests were done on 10 euthyroid unexposed Rongelap and Utirik people during a subsequent survey. The mean initial plasma T, in this group was 6.01.7 ug /dl; and the mean increment 24 hr after TSH injection was 4.21.3 ug/dl, significantly greater (p<.0.001) than in the exposed subjects. These results and the finding of elevated plasma TSHlevels suggest that there is underlying,clinically inapparent, thyroid damagein the exposed Rongelap population. Whileit is conceivable that the Ty replacement program may haveled to de- creased thyroid reserve, the test results indicate a need for continued close follow-up of the exposed