THE DIRECT MEASUREMENT OF I131UPTAKE IN THE THYROID GLAND; FURTHER OBSERVATIONS*

Abstract

A new method for the quantitative measurement of thyroid gland uptake of I131 is descr. The instrument consists of 4 Cu cathode Geiger- Mueller tubes connected in parallel and arranged horizontally in a circle, radius 45 cm. Each tube is shielded in a 2 inch-thick Pb box, a window 10 x 26 cm. remaining. The "free circle," previously descr. as a circle concentric to the outer tube circle, within which a source of I131 can be moved with a change in counting rate of less than 5%, has a radius of 10 cm. The radiation from 12.5 microcuries of I131 was equal to that recorded as background; this sensitivity allows the use of 100-150 microcuries as a tracer dose. Mock-up expts. demonstrated that the detn. of thyroid gland uptake by external counting with the 4-tube circle, after tracer and therapeutic doses of I131, is in excess by approx. 8%. The measurement, thus corrected, is accurate to [plus or minus] 5%. Studies of thyroid gland uptake after tracer doses of I131 as measured by the Pt (shielded 4-tube, Pt cathode, circle radius 60 cm.) and Cu circles showed an avg. difference of -0.25% (S.D. 3.1) in 96 comparisons in 25 patients. No correlation could be drawn between this % difference and the estimated wt. and position of the thyroid gland or the volume and configuration of the neck. Biologic half-lives were detd. from measurements in both "circles"; in 21 comparisons these differed by 5.5% (S.D. 4.3). The thyroid gland uptake as measured by the C.C.C.,following tracer and therapeutic doses of I131 was studied in 10 patients. The 24-hour uptake following therapeutic doses of I131 differed by an avg. of 7.4% (S.D. 4.2) from the % uptake 24 hrs. following tracer doses. The method here descr. offers quantitative accuracy, reduplicability and simplicity in studies of thyroid gland uptake and turnover following tracer and therapeutic doses of Il31.