INTRODUCTION The most commonly used radiopharmaceutical for in-vivo investigation of thyroid function and anatomy is sodium iodide, iodine-131. However, the radiation dose for usually administered doses is excessively high, higher than for any other radionuclide examination, Ready availability at low cost with good shelf-life accounts for its widespread use. Thyroid function tests have been standardized to its use. In an attempt to improve the quality of imaging procedures and to reduce radiation dose some have advocated the use of iodine-125 (1-3). The radiation dose from iodine-125 is still rather high and technical problems are encountered in its use for quantitative studies. More recently technetium-99m, as pertechnetate has enjoyed increasing popularity as an imaging agent (4-7) in addition to its use for functional evaluation of the thyroid (8-10). Reasons for its popularity are low cost, ready availability, high photon yield and low radiation dose. Some clinicians remain reluctant to accept it as an agent for assessment of thyroid function because it is not a natural metabolite of the gland. number of other drawbacks to its use have been noted. A On occasion there may be a difference between the technetium (trapping) and iodine (binding) image (11-16). These differences occur for a variety of reasons and are @ source of confusion. When the thyroid gland is large or substernal in | location, a poor image is obtained, Often, with low uptakes the image is not diagnostic due to the low target-nontarget ratio, 5001394