The purpose of this study is to evaluate a new diagnostic imaging test, positron emission tomography (PET), with a different radioactive form of iodine called iodine-124. This form is able to accurately measure the amount of radioactive iodine uptake in the cancer. If the new test determines sufficient radioiodine uptake in the cancer, treatment will continue as usual. However, if the new test shows only low radioiodine uptake, a decision may be made that the benefit from radioiodine therapy is insufficient and that another form of therapy is preferred.

Inclusion Criteria:

• Adults with thyroid carcinoma confirmed by pathology.
• Adult thyroid carcinoma patients who have undergone total thyroidectomy
• Adult thyroid carcinoma patients have metastatic disease or suspicion for metastatic disease, or are under a protocol intended to explore re-induction therapy for 131I uptake, and are about to undergo Thyrogen-assisted dosimetry, unless Thyrogen-assisted dosimetry is not required per standard of care.
• The patient and physician are planning to administer 131I for therapy if persistent radioiodine-avid metastases are present.
• All subjects must have measurable disease, documented within the previous six months by ultrasonography (US), MRI, FDG PET/CT, or CT scanning.

Exclusion Criteria:

• Age less than 18 years.
• Patients who are pregnant.