Endocrine Abstracts (2017) 49 EP1454 | DOI: 10.1530/endoabs.49.EP1454

Unexpectedly low thyroglobulin levels in differentiated thyroid carcinoma - case report

Francisco Norton-Brandão1, Teresa Ferreira1, Manuel Rio Carvalho1, Inês Patrocínio Carvalho1, Rita Sousa1, Pedro Ratão1, Lucília Salgado1, Joana Simões-Pereira2 & Valeriano Leite2

1Nuclear Medicine Department, Portuguese Oncology Institute - Lisbon, Lisbon, Portugal; 2Endocrinology Department, Portuguese Oncology Institute - Lisbon, Lisbon, Portugal.

Case report: 66 year old male with a neck growing mass performed an ultrasonography and fine-needle aspiration biopsy which was suspicious for papillary carcinoma. The patient was submitted to total thyroidectomy and central and left lateral compartments lymph node dissection. The histological report showed a follicular carcinoma of 9 cm, with multiple lymphovascular invasions and skeletal muscle invasion. There was a focally positive surgical margin and resected lymph nodes had no metastases. Thoracic CT showed multiple lung nodules; PET showed faint uptake in a single nodule in the upper lobe of the right lung. Five months after surgery, 5,5 GBq (150 mCi) of iodine-131 (RAI) were administered. Forty-eight hours after therapy whole-body scintigraphy (WBS) was performed: moderate thyroid remnant and faint uptake in the lower third of the right thoracic area were visible. Non-stimulated serum thyroglobulin (Tg) was undetectable and stimulated levels were 1.5 ng/ml; anti-thyroglobulin antibody (ATg) was negative. Two years after RAI, Tg levels increased to 15.2 and to 43.1 ng/ml six months thereafter. ATg remained negative. Neck and thorax CT showed a new lesion in D3 and no progression of lung nodules. The patient referred light numbness in his left arm and hand. A second RAI was performed under thyroid hormone withdrawal. Stimulated serum Tg was 229.0 ng/ml. PET-CT (Default 1) was performed just before RAI administration: uptake in lytic lesions in D3 and D4 was found (SUVmax =11.3). Post-therapeutic WBS (Default 2) also showed intense uptake in the dorsal spine and faint uptake in pulmonary lesions.

Conclusion: Serum thyroglobulin is usually useful to monitor disease progression. In patients with aggressive differentiated thyroid carcinoma, Tg alone may not reflect accurately disease progression, as in this patient with documented bone and pulmonary disease that is both differentiated and aggressive.

Fig. 1 18F-FDG PET-CT.

Fig. 2 WBS.

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