Diagnostic Contribution and Therapeutic Impact of ¹⁸F-FDG PET/CT in TENIS Syndrome

Introduction: TENIS (Thyroglobulin Elevation with Negative Iodine Scintigraphy) syndrome is characterized by serum elevated thyroglobulin despite a negative whole-body radioiodine scan in patients treated for differentiated thyroid carcinoma. This study evaluated the diagnostic performance and therapeutic impact of [18F]FDG PET/CT in patients with TENIS syndrome.
Methods: We conducted a retrospective descriptive and analytical study at the Nuclear Medicine Department of Ibn Rochd University Hospital, Casablanca, between 2021 and 2025. Patients with TENIS syndrome were included. Clinical, histopathological, biochemical, and PET/CT data were analyzed. Univariate analysis identified factors associated with PET/CT positivity, while the correlation between serum thyroglobulin and SUVmax was assessed. Therapeutic impact was classified as none, moderate (diagnostic confirmation without treatment modification), or major (change in treatment strategy, including surgery, lymph node dissection, external beam radiotherapy, systemic therapy, or discontinuation of radioiodine therapy).
Results: PET/CT was positive in 72.5% of patients. Positive findings were locoregional in 79.3% and metastatic in 51.7%, with the lungs being the most frequent metastatic site (41.4%). Serum thyroglobulin correlated significantly with SUVmax (ρ = 0.62, p < 0.001). PET/CT positivity was significantly associated with aggressive histology, histological subtype, initial metastatic disease, and higher thyroglobulin levels (p < 0.05). PET/CT modified patient management in 67.5% of cases, with a major therapeutic impact in 22.5%.
Conclusion: [18F]FDG PET/CT showed high diagnostic yield and substantial therapeutic impact in patients with TENIS syndrome, supporting its value in detecting recurrent disease and guiding individualized management. These findings should be interpreted in light of the retrospective, single-center design and relatively limited sample size.