A 63-year-old woman underwent thyroidectomy for papillary thyroid adenocarcinoma and cervical lymph node resection. Pathological analyses revealed the presence of signet cell carcinoma in a resected lymph node, which were apparently different from the pathological findings of thyroid carcinoma. No evidence of a primary tumor could be found elsewhere despite detailed examinations, including esophagogastroduodenoscopy, colonoscopy, capsule endoscopy, CT scan, and fluorodeoxyglucose-positron emission tomography. Two and half years later, the patient developed multiple bone metastases and the pathological findings confirmed the presence of signet cell carcinoma. The primary origin remained undetermined. Metastatic signet ring cell carcinoma of unknown primary origin is extremely rare.

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