Despite expert surgery, a small amount of normal thyroid tissue is commonly left in the thyroid bed following removal of a thyroid cancer. Patients are often treated with 131I after surgery to destroy the remaining thyroid. About six months later, a diagnostic whole body scan (DWBS) is done with 131I to see whether any tumor is left that still can pick up some iodine. If the scan is positive, it confirms that there is still tumor present. Unfortunately, thyroid cancer does not pick up iodine very well and so a negative test does not mean there is no thyroid tumor left. The poor resolution with 131I can mean a diagnostic whole body will not detect the tumor. Another test is becoming more widely used to detect recurrent or persistent disease. It involves injecting TSH and measuring the serum thyroglobulin level: if the level rises more than 2 ug/ml, there is a high likelihood that tumor is present. PET scanning can detect a different isotope of iodine 124I. Recent studies indicate that when a 124I PET scan is combined with a CT scan, the detection and localization of residual thyroid tumors is improved. We wish to study the CT/124I PET method for finding the spread of thyroid cancer, for estimating the size of tumors, and for calculating the dose of 131I required to eliminate the cancer.