Marked hypokalemia at presentation in an adolescent with ectopic ACTH syndrome secondary to a bronchial carcinoid tumor: a case report.

INTRODUCTION: Ectopic ACTH syndrome (EAS) is a rare cause of Cushing's syndrome in children and adolescents, accounting for less than 1% of pediatric cases. Compared with Cushing's disease, EAS follows a more rapid clinical course with more pronounced metabolic disturbances, among which hypokalemia is one of the most distinctive features. CASE PRESENTATION: A 14.7-year-old male presented with a three-month history of rapid weight gain, violaceous striae, hypertension, and proximal muscle weakness. Marked hypokalemia (2.8 mmol/L) prompted endocrine work-up, which revealed ACTH-dependent hypercortisolism with loss of diurnal cortisol rhythm, elevated midnight salivary cortisol, and non-suppression on low- and high-dose dexamethasone suppression tests. Pituitary MRI was unremarkable, and the marked hypokalemia raised suspicion of an ectopic source. Thoracic CT identified a nodular lesion in the right middle lobe, with corresponding uptake on ⁶⁸Ga-DOTATATE PET/CT. Right middle lobectomy was performed, and histopathology confirmed a typical bronchial carcinoid tumor. Postoperatively, hypercortisolism resolved, electrolyte balance normalized, and Cushingoid features regressed. CONCLUSION: Marked hypokalemia at presentation may serve as an early biochemical clue to ectopic ACTH secretion in pediatric ACTH-dependent hypercortisolism, and should prompt timely evaluation for extrapituitary sources, including bronchial carcinoid tumors.