unoperated) Rongelap populations, the prevalence of serum TSH >3 U/ml was 26%, more than twice that in the two other populations, suggesting the presence of some degree of thyroid dysfunction in this group. The occurrence of 10% of control values >3 w/ml for serum TSH concentra- tion and the occurrence of only one value >5 w/ml in either the control or the exposed Utirik populations justified the selection of 6 WU/ml as a tentative upper limit of normal. A minimum of two separate determinations of serum TSH in each individual was established as an additional requirement. Recent studies of normal individuals (in the U.S.) using this TSH immunoassay indicated that a persistent serum TSH of 6 W/ml for 6 hours achieved by constant TRH infusion results in significant increases in serum T3 and Ty, over this time period (132). Therefore a serum TSH concentration >6 W/ml as measured by this TSH assay represents a biologically significant elevation. Of 67 control and 101 Utirik exposed individuals assayed at least twice, only one had a serum TSH concentration >6 W/ml (Table 6). This 55-year-old woman (subject No. 982) had serum TSH values of 6.3 and 6.5 wW/ml and in 1978 had a serum Ty of 6.4 Ug/dl with a TBGI of 0.76 (normal range = 0.85 to 1.10). Tests for antimicrosomal and antithyroglobulin antibodies were negative. These criteria for thyroid dysfunction were clearly biochemical since none of the patients had clinical hypothyroidism. The results were, however, specific enough to permit classification and eliminate errors that might be due to variations in the assay and/or physiological variations. When these more specific criteria were applied to the exposed Rongelap populations (Table 6, second series), there were still eight individuals with TSH values >6 HU/ml on two occasions. Other pertinent observations on these individuals are presented in Table 7. The first two individuals listed in Table 7 (Nos. 3 and 5) were exposed at age 1 year (thyroid doses 1150 rads or more). Representative serum TSH values may not be maximal, especially in the case of subject No. 3, since he generally adhered to the recommendations regarding thyroid replacement medication, which was never intentionally stopped in these two. Subject No. 5, however, often had substantial elevations in TSH and markedly reduced serum Ty, indicating that he did not take thyroxine regularly. In the remaining five individuals in Table 7, serum TSH values between 6 and 10 WW/ml were obtained on various occasions. In most, the serum Ty or estimated free thyroxine index (obtained by multiplying the serum Ty by the TBGI, normal range 4.7 to 10.6) was in the low or low-normal range, as noted in other individuals with mild thyroid hypofunction (111). Thus, while subjects No. 34 and 78 had serum Ty, concentrations within the normal range, they both had a subnormal TBGI, indicating that the concentrations of unoccupied TBG binding sites were elevated in these sera, Since the normal range for serum Ty, is dependent on the quantity of circulating TBG, such subjects should have a higher serum Tg. All of the individuals with mild biochemical hypothyroidism listed in this table were exposed between the ages of 25 and 45 years and are now aged 50 to 70. To determine whether increased age could be associated with an increase in serum TSH, as observed in other surveys (133), and exposed Utirik patients in this age group ual (No. 982) was found to be abnormal out of two-thirds of these subjects had at least two none of the remaining had a TSH concentration - 71- serum TSH values in control were examined. Only one individa total of 53 tested. About serum TSH determinations, and >4 W/ml. Therefore the