The thyroid traps the pertechnetate ion and iodine in a similar manner. Thus, 99mTc-pertechnetate may be used for thyroid uptake studies (including stimulation and suppression tests), as well as imaging. Images are obtained after an intravenous administration of 2-4 mCi of 99mTc-pertechnetate, and the scan is preceded by a trapping index at 20 minutes, measured as the neck to thigh ratio. In 1973, we compared 99mTc-pertechnetate with 131I in diagnostic uptake and imaging studies of the thyroid gland in 100 patients. We have had experience with an additional 100 patients, and modified our procedure so that the Anger camera with pinhole collimator will adequately replace the standard scintillation probe. Using the camera alone, with the 99mTc-pertechnetate makes the study less expensive and briefer, with no loss of information. Routine use of the scan aids in distinguishing diffuse from localized diseases.