Use of neutron activation analysis for studying stable iodide uptake by the thyroid.

Measurement of the rate of accumulation of radioiodine by the thyroid gland is of considerable clinical value, since in most instances an avid uptake of radioiodine may be correctly assumed to be due to a high rate of uptake of stable iodide by the thyroid. This measurement, expressed as a percentage of the administered radioiodine dose accumulated per unit time by the thyroid, does not give a quantitative measurement of the stable inorganic iodide being accumulated by the gland. Occasionally an avidity for radioiodine may be attributable to an abnormally small extrathyroidal iodide pool, as in the case of the patients with endemic goiter described in the classic report of Stanbury and his associates (1). In 1949 Stanley described a method for the measurement of stable iodide accumulation by the thyroid gland using the radioiodine uptake of the thyroid and the specific activity of simultaneously excreted urine; i.e., the ratio of radioactive to stable iodide in the urine (2). The technique extended the information obtainable by the study of radioiodine accumulation alone. Technical difficulties associated with the chemical measurement of trace quantities of stable inorganic iodide have prevented its widespread application to studies of thyroid disease. The purpose of this paper is 1) to describe a new technique for the accurate and sensitive measurement of iodide in biological fluids, and 2) to present the results of its use in studies of the relationship between plasma inorganic iodide concentration and iodide accumulation by the human thyroid gland.