A Young Woman with Paraneoplastic Cushing’s Syndrome Due to a Pulmonary Carcinoid

<b>Background and Clinical Significance:</b> Ectopic ACTH secretion is a rare, potentially life-threatening cause of Cushing's syndrome that can be overlooked when small neuroendocrine tumors evade standard imaging. <b>Case Presentation:</b> A 34-year-old woman presented with rapidly progressing clinical signs/symptoms of Cushing's syndrome and demonstrated marked hypercortisolism (cortisol 2428 nmol/L; ACTH 163 ng/mL; urinary free cortisol 815 μg/24 h; K+ 2.4 mmol/L). Small hypermetabolic nodules were noted in her right lung on ¹⁸F-FDG PET/CT but were initially deemed to be infectious; DOTANOC PET-CT and inferior petrosal sinus sampling were non-diagnostic. After medically induced inhibition of cortisol, repeat PET/CT showed a persistent 13 mm lung nodule. Biopsy confirmed a well-differentiated pulmonary carcinoid (Ki-67 3%), and lobectomy achieved biochemical remission. <b>Conclusions:</b> Diagnostic delay stemmed from human factors despite early suggestive imaging. Ectopic ACTH secretion should remain high on the differential diagnosis in rapidly evolving, severe ACTH-dependent Cushing's disease; early, decisive diagnosis and coordinated care overseen by endocrinologists-preferably in expert centers-can shorten exposure to deleteriously high cortisol levels and improve outcomes.