~&- rarely would the error be of any significance. A variety of techniques utilizing the scanner (8,21) or camera (9,20,22) have been devised to determine uptake. The level of administered radioactivity has varied from one institution to another. We prefer to use about 2.5 - 3.5 mCi. The radiation dose from 99™Tc to the thyroid is about 0.3 rad/mCi. The whole body dose is somewhat more significant than with radioiodine, being about 0.013 rad/mCi (23). This should be comparedwith a radiation dose of about 1 rad/100pCi to the thyroid and about 3 mrad/1L0OpCi whole body dose. This assumes a pure product. Even a small amount of longer-lived contam- inents can double or treble the radiation dose. The normal range of technetium uptake in the thyroid in our studies has been from 0.5 to 3.75%. This compares well with other series using imaging techniques for quantitation of uptakes. Others have found higher uptakes probably because of difficulties with high circulating background and salivary gland activity. We have been looking at the clinical utilization of iodine-123, not only in comparison with icdine-131 for conventional uptake studies, but also in comparison with technetium-99m for trapping studies, in addition to comparing the imaging qualities of both radionuclides. A previous pub- lication (15) indicated some differences in quality of images obtained with 2™f¢ and 1237, Biological differences between the "trapping phase" image at 30 minutes and the "binding phase" image at 24 hours were noted in a few instances. / The radioiodine image is superior when uptake is very low (Fig. 2), in the presence of substernal extension (Fig. 3) and for detection of nodules alongthe border of the gland (Fig. 4). In several instances | functioning nodules seen on the pertechnetate image were either normally 590139 |