Serum Pratein-Bound Iodine—Rall and Conard asymptotic uptake is normal or slightly elevated. These data together with the urinary iodine values, may be used to calculate the average daily secretion of thyroid hormone, assuming steady state conditions, by the formula S$ = > where 5 amount of iodine secreted by the thyroid (micrograms per day) U = fractional thyroid uptake of iodine E = urinary iodine (microgramsper day) Using E = 105 ug. per day and U = 0.42, an average value for 5 may be calculated to be 76 ug. of iodine per day. This value is somewhat higher than similar ones calculated for other groups but is not extraordinarily high (70-72]. COMMENTS It has not been possible to determine the basal metabolic rate in the inhabitants of Rongelap. However, the consensus of all phy- sicians who have examined these people is that they are not hyperthyroid. The explanation for the large number of subjects with a high PBI level is, therefore, surely not an epidemic of hyperthyroidism. An elevation of thyroxine-binding proteins in serum could, as in the cases of congenital elevation of thyroxine-binding globulin described by Beier- waltes and Robbins, cause an increase in serum PBI without hyperthyroidism [73]. The serum levels of TBG in the Marshallese measured by Dr. J. Robbins were, however, within normal limits. The discrepancy between PBI and BEI, however, suggested the presence of an iodoprotein in serum. The chromatography of serum iodine showing an iodoprotein level in the Marshallese of 2.2 ug. per cent seems to implicate the iodoprotein as associated with the elevated PBI level. Detailed data are not available on the calorigenic potency of serum iodoproteins but some results show that most of the iodinated amino acids in this protein are monoiodotyrosine and diiodotyrosine [74,75]. These iodoamino acids are devoid of physiologic activity. Hence an iodoprotein containing only these iodoamino acids is likely also to be physiologically inactive. The reason these persons secrete such an iodoprotein into the blood is not at all clear. The data on normal control subjects from the Eastern United States, who showed 0.8 ug. VOL. 40, JUNE 1966 885 TABLE tI KINETIC STUDIES WITH Gases An* : dat Theoretical Uptake Marshallese | Normal 0.72 1.0 0.97 2.0 . 42% 33% wat No. Cases 0.08 as 21} eae * da is the fraction of extrathyroidal iodide transferred to the thyroid per day. : + Aa: is the fraction of extrathyroidal iodide excreted in the urine per day. toais the value derived by the computer for the fraction of extrathyroidal iodide “seen” by the counter. per cent iodoprotein in their serum, suggest that it is a normal albeit minor constituent. The method of chromatography employed is such that well under 5 per cent of serum thyroxine (or 0.2 pg. per cent) appears in the unretarded or iodoprotein fraction. Therefore, the finding of iodoprotein does not appear to be a methodologic artifact. The urinary iodine values were in the normal range. In general it had been expected that subjects who live close to the sea and who eat seafood and fish would have a relatively higher iodine intake. The inhabitants of the Marshail Islands have fish as one of their main sources of animal protein. Furthermore, these people are constantly exposed to sea spray since the highest point on the atoll is approximately 20 feet above high tide and the island at its widest is about a quarter of a mile across. The data on urinary iodine were used with the results obtained with I?* studies to calculate the amountof iodine secreted daily by the thyroid. The value of 76 yg. is somewhat higher than the figures of 57 wg. per day found by Stanbury e¢ al. [70] or 58 yg. per day found by Freinkel and Ingbar [77], but closer to the value of 70 yg. per day proposed by Riggs {72]. Unfortunately, nothing is known about the rate of turnover of the serum iodoprotein. If it has roughly the same rate of degradation and the same volume of distribution as thyroxine, we would expect the thyroidal secretion of organic iodine in the Marshallese to be proportional to the level of organic iodine in their serum. Adding iodothyronine values to iodoprotein levels for both Marshallese and Americans, and multiplying the ratio by the best value for iodine secreted by normal Americans we obtain: 2.22 + 4.53 T 59 = 86 ue. da 0.80 13.76 * HG. per Cay 4 ! j b { a 3 $ é j # * r g ié é t , € ‘ ' \ 5Uf2524