Distant metastases is the leading cause of differentiated thyroid cancer-related death. Radioactive iodine (RAI) treatment is the most effective therapy for RAI-avid metastatic differentiated thyroid cancer (DTC). It is well known that the efficacy of RAI therapy is depend on the sodium-iodide symporter protein, which can be synthesized by elevated thyrotropin stimulation. Therefore, thyrotropin stimulation before RAI treatment to ensure sufficient uptake of RAI has been a long-established procedure. According to some observational studies, thyrotropin of 25-30 μIU/mL has been adopted as the standard care protocol. However, whether thyrotropin of 25-30 μIU/mL is enough to stimulate iodine uptake in metastatic lesions remains unknown. In this study, the investigators aim to address the effect of thyrotropin on iodine uptake in metastatic DTC during levothyroxine withdrawal by two times 124I PET/CT scans on different endogenous thyrotropin levels.

It is estimated that 10 patients who met the study criteria will be enrolled. All patients underwent twice 124I PET/CT scans in Beijing Cancer Hospital. The first 124I PET/CT was prepared with endogenous thyrotropin 30±10 μIU/mL, the second 124I PET/CT was prepared with endogenous thyrotropin >100 μIU/mL. The RAI-avidity of the metastases was assessed twice along with the levothyroxine withdrawal by 124I PET/CT scans.

Inclusion Criteria:

1. Patients underwent total thyroidectomy.
2. Patients were diagnosed with differentiated thyroid cancer by postoperative pathological examination.
3. Distant metastases confirmed by CT etc.

Exclusion Criteria:

1. Pregnancy or lactation.
2. Significant hepatic or renal dysfunction;
3. Unable to lie flat, still or tolerate a PET scan.