Background: This study aimed to determine the role of fluorine-18-fluorodeoxyglucose positron emission tomography (FDG-PET) in the follow-up of patients who underwent total thyroidectomy and iodine-131 (¹³¹I) ablation therapy for differentiated thyroid cancer and presented increased thyroglobulin levels with negative ¹³¹I and thallium-201 (²⁰¹Tl) scans.

Methods: Two patients with follicular carcinoma and eight with papillary tumors underwent total thyroidectomy and ¹³¹I therapy until the ¹³¹I scan was negative. ¹³¹I and ²⁰¹Tl scans were performed with negative results in all cases, while serum thyroglobulin measurements were all positive with negative thyroglobulin autoantibodies. One week after the ¹³¹I scans, all the patients underwent FDG-PET whole-body scans.

Results: The FDG-PET scan detected in 4 patients, a single focal increase of FDG uptake in one lymph node metastasis (subsequently confirmed histologically); in 1 patient, multiple pathological focal uptakes in brain, neck, and chest; and in 1 patient, two mild focal uptakes in the mediastinum, close to the tracheal branch. In 2 other patients, pathological FDG uptakes in cervical spine and mediastinum were not confirmed by other imaging techniques, and in the 2 remaining patients the scan results were inconclusive. The sensitivity of FDG-PET whole-body scan for detecting metastatic thyroid cancer was 60%.

Conclusions: This study indicates that the FDG-PET whole-body scan is a useful tool in the follow-up of patients with differentiated thyroid cancer, negative ¹³¹I and ²⁰¹Tl scans and elevated serum thyroglobulin levels. The FDG-PET scan detects metastatic disease in 60% of patients with differentiated thyroid cancer, enabling surgical therapy to be performed on accessible lesions.