33 who hadbeen exposed at age 10 to 20 years, gave results similar to those obtained in 1965. Thus the Rongelapese do not show a trend toward decreasing thyroid iodine uptake as do North Americans (attributed to increased dietary iodineintake). The main purpose ofthelater study, how- ever, was to see whether the exposed individuals in the intermediate age group might be developing thyroid failure despite the absence of thyroid nodules; its results indicated no evidenceforthis. (However, as discussed below. some exposed Rongelap people without thyroid abnormalities are now showing evidence of reduced function on the basis of response to TSH stimulation.) 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 surgery. The results on subjects with knownthyroid lesions are presented in Table 27. Of the 32 subjects tested, 17 or 33% had at least one TSH level above the upper limits of normal (these are in boldface in Table 27). These findings indicate thatthe residual thyroid tissue is inadequate to sustain euthyroidism in these cases and also reflect inadequate adherence to the prescribed Ty replacement regimen. Inadequate T4 replacementis apparently a chronic problem for certain patients (Nos. 5, 23, 33, 65, and 72). Ele- vations of plasma TSH have been observedalso in a numberof exposed people without known thyroid lesions (the TSH concentration was > 10 uU/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 con- centration >5 but <10 hU/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 Ty <5 ug/ 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 TSHlevel (No. 2232). These normal findings in the unexposed and Utirik groups suggest that in the irradiated Rongelap group thereis impaired thyroid function without palpable lesions which could become symptomatic in the future. The test results prob- ably lead to underestimation of the true incidence of impaired thyroid function, since presumably manyofthe patients are taking the medication as directed. It should be noted thatit is the personal experience of manyof 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, whichis now recognized as the most sensitive indicator ofprimary thyroid dysfunction. The status of thyroid function in exposed people without apparentthyroid lesions wasfurthertested with exogenous TSH in 1974, as described below. 3. Thyroid Status of Exposed Rongelap People Without ApparentThyroid Lesions In preparation for TSH testing of reserve thy- roid function, prophylactic T4 medication was discontinued for 2 months before the 1974 survey in all exposed subjects without recognized lesions. During the survey, plasma samples were obtained before and 24 hr after intramuscularinjection of 10 units of bovine TSH (Thyrotropar, Armour). Both sets were analyzed for Ty and thefirst set also for TSH. Theresults, and thyroxine-binding globulin-binding capacities (TBG-binding capacities} in some cases, are given in Table 28. The mean incrementin plasma T4 following TSH was 2.35 1.2 pg/di (mean + S.D.). The mean T4 prior to TSH injection was 6.6£1.7 wg/dl. Similar tests88 on 13 subjects at the University of Pittsburgh showed a mean incrementin plasma T4 of 4.7 1.0 wg/dl, and a baseline plasma T, of 7.3 ug/dl whichis not significantly different from thatof the exposed Rongelap group being tested. Thus, the T4 response to TSH is significantly less (p<0.001) in this group of 26 exposed Rongelap subjects than in the group of 13 subjects from the United States. Because of the possibility that the smaller incrementin plasma T, 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 survey. The meaninitial plasma T, in this group was 6.0+1.7 ug/dl; and the mean increment 24 hr after TSH injection was 4.21.3 ug/dl, significantly greater (<0.001) than in the exposed subjects. These results and the finding of elevated plasma TSH levels suggest that there is underlying, clinically inapparent, thyroid damagein the exposed Rongelap population. While it is conceivable that the T, replacement program mayhaveled to de- creased thyroid reserve, the test results indicate a need for continuedclose follow-up of the exposed