Patients will receive two additional injections of rhTSH. This is similar to the procedure for I-131 dosimetry. Second, they will receive I-124. I-124 is similar to I-131 except I-124 decays in a different way to emit a positron so that the PET scanner can be used for imaging. I-124 is given in the form of one or several capsules, which are taken by mouth. This is also similar to I-131. Third, PET/CT imaging is done for approximately 30 minutes to one hour on five consecutive days. Radiation from PET/CT scan is far less than what they receive from a diagnostic CT scan. For the fourth part, a technologist will draw about 5 cc from the forearm on each of the five consecutive days. This is also similar to I-131.