Background <p>Neoplasms arising simultaneously from the thyroid, parathyroid, and paraganglionic tissues are exceptionally rare. To our knowledge, no previous case has documented the synchronous occurrence of poorly differentiated thyroid carcinoma, parathyroid adenoma, and cervical paraganglioma. This report highlights the diagnostic and embryologic significance of this rare triad.</p> Case presentation <p>A 79-year-old South Asian woman presented with a slowly progressive anterior neck swelling. Imaging showed a mass in the left thyroid lobe with vascular displacement. Total thyroidectomy with neck dissection was performed. Histology revealed poorly differentiated thyroid carcinoma with insular architecture and high mitotic activity. Adjacent to the tumor, a parathyroid adenoma was identified. A separate encapsulated lesion near the internal jugular vein showed a Zellballen pattern consistent with paraganglioma. Immunohistochemistry confirmed all three components. There was no history of familial endocrine neoplasia.</p> Conclusion <p>This case presents a unique triad of poorly differentiated thyroid carcinoma, parathyroid adenoma, and paraganglioma—an association not previously reported. Their coexistence in a single patient may reflect a shared embryologic origin from neural crest derivatives. Recognition of such unusual combinations can aid in accurate diagnosis, prevent misinterpretation of tumor margins, and guide appropriate surgical and follow-up decisions.</p>