Metastatic Papillary Thyroid Carcinoma Without Intrathyroidal Primary: Case Report and Literature Review.

Papillary thyroid carcinoma (PTC) usually has an intrathyroidal primary lesion and a good prognosis. In rare cases, however, individuals may also present with metastatic disease in the absence of a palpable thyroid primary, so-called occult PTC. Such cases are often difficult to diagnose and manage, especially when serial monitoring of serum thyroglobulin (Tg) levels and imaging studies demonstrate the opposite. We report the case of a 56-year-old woman who underwent total thyroidectomy for suspected multinodular goiter in 2019. The results of a histological examination revealed no intrathyroidal tumor in 78 blocks, but one central node was found to be positive for metastatic PTC. Follow-up examination proved that Tg had increased even though the results from U/S, PET/CT, and I-123 WB scans were always negative. Laboratory studies suggested potential heterophile antibody interference, but a fine needle aspiration (FNA) of a cervical node in 2023 and 2024 reported recurrence of metastatic PTC. She underwent left selective lateral neck dissection (levels II-V) with central compartment (level VI) exploration in January 2025, and a number of metastatic nodes without extranodal extension were found. Tg levels were undetectable throughout follow-up with no evidence of residual disease on US postoperatively. The patient remains well on long-term follow-up. We present a rare case of metastatic PTC with no identifiable thyroid primary, which is combined with the diagnostic difficulties of Tg assay interference and limitations of imaging in the follow-up setting. It highlights the importance of multimodal surveillance (including cytology, imaging, and biochemical testing) to ensure true recurrences are accurately detected, often many years after apparently curative surgery.