Thymoma Masquerading as Cushing Syndrome: A Rare Case of Ectopic ACTH Secretion from Atypical Thymic Carcinoid (Grade 2 NEC)

Background: Ectopic adrenocorticotropic hormone (ACTH) secretion (EAS) accounts for 9%–18% of ACTH-dependent Cushing's syndrome (CS) cases. Thymic neuroendocrine tumours (NETs), comprising only 2%–5% of all thymic neoplasms, represent a rare but clinically significant cause of EAS. We present an unusual case where an atypical thymic carcinoid (Grade 2 neuroendocrine carcinoma) presented as florid Cushing syndrome, complicated by opportunistic infection and severe end-organ manifestations. Case Presentation: A 54-year-old male presented with an 18-month history of bilateral facial puffiness, progressive proximal muscle weakness, hyperpigmentation of the lower limbs, and right-sided visual loss. Biochemical evaluation confirmed severe ACTHdependent hypercortisolism with non-suppressible cortisol, markedly elevated plasma ACTH (>3× upper limit of normal), and hypokalemic metabolic alkalosis. Imaging revealed a 5.2 × 4.0 cm heterogeneously enhancing anterior mediastinal mass. Ga-68 DOTATATE PET-CT demonstrated intense tracer avidity (SUVmax 14.6), confirming somatostatin receptor expression. The patient underwent successful R0 thymectomy via median sternotomy. Histopathology confirmed an atypical thymic carcinoid (Grade 2 NEC) with ACTH, synaptophysin, and chromogranin A positivity. Conclusion: This case highlights the diagnostic challenge of ectopic ACTH-secreting thymic NETs, which can mimic pituitary-dependent Cushing disease. A high index of clinical suspicion, systematic biochemical workup, and functional imaging (Ga-68 DOTATATE PET-CT) are essential. Complete R0 surgical resection remains the cornerstone of curative management.