I-131 false-positive uptake in a thymic cyst with expression of the sodium-iodide symporter: A case report

Medicine (Baltimore). 2022 Jul 1;101(26):e29282. doi: 10.1097/MD.0000000000029282.


Rationale: I-131 radioiodine false-positive findings in postoperative patients with differentiated thyroid cancer (DTC) should be recognized to avoid unnecessary therapies.

Patient concerns and diagnoses: A 50-year-old man underwent I-131 therapy 3 times, including the initial ablative therapy after total thyroidectomy for papillary thyroid cancer. The initial I-131 posttherapeutic whole-body scintigraphy showed 2 cervical and one superior mediastinal focal I-131 positive uptake lesions. The serum thyroglobulin level was negative every time when the radioiodine therapy was performed. Although the 2 cervical positive uptake lesions disappeared after the second therapy, the superior mediastinal I-131 positive uptake persisted even after the third therapy, and this lesion was suspicion of I-131 therapy-resistant node metastasis.

Interventions and outcomes: The lesion was resected, and the pathological diagnosis with immune-histochemical analysis was a thymic cyst with thymic epithelial cells having a weak expression of the sodium-iodide symporter (NIS).

Lessons: The false-positive result may be attributed to the NIS expression in the thymic cyst epithelial cells. It is necessary to include a thymic cyst in the differential diagnosis, when I-131 uptake is noted in the superior mediastinal region on I-131 posttherapeutic scans of patients with postoperative DTC. Although the I-131 positive uptake in a thymic cyst may be influenced by the I-131 administered dose and scan timing after I-131 administration, the NIS expression may be essential to the false-positive uptake in a thymic cyst.

Publication types

  • Case Reports

MeSH terms

  • Adenocarcinoma*
  • Humans
  • Iodine Radioisotopes / metabolism
  • Iodine Radioisotopes / therapeutic use
  • Male
  • Mediastinal Cyst*
  • Middle Aged
  • Symporters
  • Thyroid Neoplasms* / diagnostic imaging
  • Thyroid Neoplasms* / surgery
  • Tomography, X-Ray Computed


  • Iodine Radioisotopes
  • Iodine-131
  • Symporters
  • sodium-iodide symporter