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Background: Cushing's syndrome (CS) due to ectopic adrenocorticotrophic hormone (ACTH) and/or ectopic corticotropic releasing hormone (CRH) secretion accounts for <10% of all CS. Neuroendocrine carcinomas rarely cause CS. These carcinomas have been found to secrete either ACTH or rarely CRH. Herein we report a case of neuroendocrine carcinoma originating from the thyroid as the source of ACTH-dependent CS. Summary: A 30-year-old woman with features of CS presented with severe respiratory distress. Six months before that, she was diagnosed with primary hypothyroidism and started on levothyroxine (LT4) therapy. Biochemical evaluation was done, and nonsuppressed serum cortisol levels following dexamethasone with high ACTH confirmed a diagnosis of ACTH-dependent CS. Magnetic resonance imaging of the brain showed a bulky pituitary gland. Adrenal imaging showed bilateral adrenal hyperplasia. A computerized tomography scan showed a large anterior mediastinal mass arising from the neck and extending behind the transverse aortic arch. She underwent emergency thoracotomy due to rapidly progressive superior mediastinal syndrome and left vocal cord palsy. At surgery, the mass was seen originating from the thyroid and the thymus was compressed posteriorly. Near total thyroidectomy and thymectomy with removal of pericardial seedlings were done. Histopathology revealed sheets, cords, and nests of round or oval tumor cells with hyperchromatic nuclei and scant cytoplasm with local invasion and lymphovascular embolization suggestive of a neuroendocrine carcinoma arising from thyroid, staining positive for cytokeratin, synaptophysin, and chromogranin-A, and negative for calcitonin and carcinoembryonic antigen. Conclusions: Here we report a case of a neuroendocrine tumor of the thyroid causing ACTH-dependent CS. The tumor was negative for calcitonin staining, indicating that this was not a medullary carcinoma of the thyroid. Neuroendocrine carcinomas originating from the thyroid gland are very rare. A thyroid tumor of neuroendocrine origin causing ACTH-dependent CS has not been reported previously. [ABSTRACT FROM AUTHOR]