IAEA-SM-224/607 107 function which are described below. Occasionally, this instruction was not followed. However, the nature of the tests performed was such that this circumstance would result in an underestimation (rather than an overestimation) of the frequency of abnormalities in thyroid function. Plasma was separated by allowing the red cells to sediment and was frozen within eight hours after obtaining the specimen. Serum Thyroid Stimulating Hormone (TSH) and thyroxine (T4) were measured as previously described [8]. The normal range for serum TSH in ourlaboratory is from <0.05 to 3 wU/ml in the United States population (mean 2.0 nU/ml). Thyrotropin Releasing Hormone (TRH)stimulation tests were performed by infusion of 500 yg of TRHintravenously, plasma samples were obtained at O and at 20 minutesafter the infusion. TSH stimulation tests were performed by administration of ten units of bovine TSH intramuscularly with plasma obtained for T, determinations before injection and 24 hours later. The normal range for serum T, concentrations is 5-10 yg/dl. Estimation of the free fraction of T, was obtained by aT; charcoal uptake method developed in our laboratory (TBG Index). Twenty-five ul of plasma are incubated in | ml of glycine acetate buffer, pH 8.6, containing 1251 T,. Dextran-coated charcoal is added at 4°C with subsequent centrifugation to sediment the charcoal. The fraction of the total 1251 T, bound to charcoal ‘is determined andthis result is normalized to the results of simultaneously assayed quality control samples containing normal quantities of T, and thyroxine-binding globulin (TBG). The normalrange for the test is 0.85—1.10. The TBG Index increases parallel to the free fraction of the serum T, and T; and is therefore elevated in hyperthyroidism or TBG deficiency. RESULTS TSH stimulation tests Somer To determine whether or not there was impaired thyroid reserve in the exposed subjects, TSH stimulation tests were carried out using an increase in serum T, as the response endpoint. In normal subjects in the United States of America, the mean increment in plasma T, was 4.7 + | yg/dl (mean + SD) in 13 subjects following injection of 10 U of TSH. In Table III are shown data for the Marshall Islands population. The control subjects who had not been exposed to radiation were given TSH, and the mean increment in T, was 4.2 + 1.3 (SD) not statistically different from the results in the United States population. However, in 24 exposed Rongelap subjects, a mean increment of only 2.4 + 1.2 ug/dl was obtained, which wassignificantly less (p < 0.001) than in the control subjects. goTE OGY 12504