THE EXCHANGEABLE THYROID HORMONAL POOL. I. ITS MAGNITUDE AND RATE OF TURNOVER IN VARIOUS THYROID STATES IN MAN 12

Abstract

The turnover rate of human thyroid hormone has been studied by following the disappearance of plasma I131 obtained from thyrotoxic patients who had received 10.3 to 23.1 mc of radioactive iodine, and infused into 4 euthyroid patients, 2 with thyrotoxicosis and 1 with myxedema. The 2d or slow exponential decrease in radioactivity has been considered to represent the replacement of tagged hormone or its turnover. The half-times in euthyroid subjects were 50.2-64.5 hrs., in contrast to 39.5 and 34.0 hrs. in the patients with thyrotoxicosis, and 41.2 hrs. in the myxedematous patient. The turnover rates were 25.7-33.2%/day in the euthyroids vs. 42 and 49%/day in the thyrotoxics and 34%/day in the myxedematous patient. The exchangeable and circulating hormonal pool have been estimated by the principle of isotope dilution and from the former the daily turnover has been calculated. The exchangeable hormone pool ([mu]g-) averaged 399 [plus or minus] 44 in the 4 euthyroid subjects vs. 1142 and 648 in the thyrotoxic patients and 65 in the myxedematous patient. The daily turnover ([mu]g. per day) averaged 119 [plus or minus] 11 in the 4 euthyroid subjects vs. 480 and 317 in the thyrotoxic patients and 26 in the myxedematous patient. The method of study used permits delineation of another parameter of thyroid function; its application to varied aspects of thyroid physiology and pathology appears warranted.