MEASUREMENT OF THE ACTIVE IODINE STORES AND DAILY HORMONAL OUTPUT OF THE INTACT HUMAN THYROID*

BAUMANN (1) was the first to recognize that the thyroid gland had the capacity to retain and store iodine. Other workers (2, 3) have shown a positive correlation between functional states of the thyroid gland and thyroidal iodine stores. Since these studies were performed in glands which were removed at operation or at the autopsy table, the data so derived were not applicable to the living patient except in a general way. It is apparent that an experimental method which would allow such studies in vivo would be extremely helpful, both in understanding the pathogenesis of thyroid disease and in helping to determine the type and amount of therapy required. Tracer doses of radioactive iodine which are fixed in the thyroid gland can be released into the blood stream along with stable hormonal iodine in response to administered thyroid-stimulating hormone (TSH). By comparing the specific radioactivity of the newly secreted hormone with the thyroidal content of I131, it is possible to estimate the hormonal stores of the intact human gland. Becker et al. (4) employed this technique in 3 eu thyroid subjects and assumed that after five to eight days the specific activity of the circulating serum protein-bound iodine (PBI) was equal to that of the thyoidal compartment. In the present study the specific radioactivity of the newly secreted PBI was determined directly, so that equilibration of thyroidal and extrathyroidal PBI compartments was not necessary for valid hormonal store calculations.