Measurement of Radioisotope Uptake in Thyroid

This paper reviews methods for the measurement of radioactivity in the thyroid. The main problems arising in such measurements are shown to be uncertainty regarding the depth of the thyroid in the neck tissues and uncertainty regarding the contribution of extra-thyroidal radioactivity to the observed counting rate. The radioisotopes ⁹⁹Tc^m, ¹²³I, ¹²⁵I, ¹³¹I and ¹³²I are all employed in the investigation of thyroid disorders but ¹³¹I is the most widely used of these. The various methods in current use for the measurement of this radioisotope in the thyroid are described; these include methods based on the use of single fixed detectors, dual fixed detectors and area scanning systems. Techniques for determining the effective depth of the thyroid include the making of measurements with a single fixed detector at two different distances and the making of measurements on two groups of photons differing in energy. Techniques for correcting the measurements for the contribution of extra-thyroidal radioactivity include the making of measurements over the thigh and the making of measurements with a lead shield covering the thyroid. (author)