differential diagnosis of hypokalemia and adrenocorticotropic hormone ( acth)-dependent cushing 's syndrome often presents challenging in endocrinology and requires careful clinical , biochemical , radiological , and pathological investigations . hypokalemia is a common abnormality and systematic approach is required to avoid delays in diagnosis of important underlying causes . after excision biopsy , all of the patient 's symptoms improved and electrolytes and acth levels also became normal . carciniod tumors should be considered as a differential diagnosis in patients presenting with hypokalemia and ectopic acth syndrome . hypokalemia associated with metabolic alkalosis and hypertension is highly suggestive for overproduction of adrenocorticotropic hormone ( acth ) . cushing 's syndrome due to cancer was first described in 1928 in a patient with small cell carcinoma of the lung . in the 1960s various tumors can secret ectopic acth that small cell lung carcinoma , carcinoid tumors ( especially of the lungs , thymus , and gastrointestinal tract ) , islet cell tumors , pheochromocytoma , and medullary thyroid carcinomas are the most frequent causes . we describe a case with carcinoid tumor in lung periphery producing acth with unusual biochemical and imaging finding . a 49-year - old woman with past medical history of diabetes mellitus and hypertension was admitted in the general internal medicine ward with chief complaint of abdominal pain , constipation , and malaise for 5 days . she also had history of unexplained weight gain since 2 months ago and denied any history of taking exogenous steroid or herbal medications . physical examination showed a body mass index ( bmi ) of 32  kg / m , blood pressure equal to 140/90  mm  hg ( on anti - hypertensive medications ) , abdominal obesity , decreased bowel sounds , and proximal muscle weakness of upper and lower extremities . routine laboratory results revealed modest hypokalemia ( 2.7  mmol / l ) , hyperglycemia ( 245  mg / dl ) , and a metabolic alkalosis . twenty four hours urine free cortisol excretion showed significant elevation ( 403  gr/24  h  normal range : 10100 ) and overnight ( 1  mg ) dexamethasone suppression test revealed a failure to suppress cortisol level ( 63.7  gr / dl    2.3 the serum acth level was elevated ( 165  pg / ml  normal range : 8.357.8  pg / ml ) . cortisol level suppressed more than 50% with high dose dexamethasone test after 2  mg dexamethasone every 6  hour for 2 days ( baseline cortisol level of 65  gr / dl decreased to 18.5  gr / dl ) . hypothalamopituitary axis magnetic resonance imaging ( mri ) with and without contrast was normal . to find the possible source of ectopic acth secretion , the spiral chest and abdominal and pelvic computed tomography ( ct ) scan requested the finding was only a 17    15  mm solitary peripheral pulmonary nodule in medial aspect of base of right lower lobe adjacent to the diaphragm ( fig . 1 ) . chest computed tomography ( ct ) scan demonstrating a small 17    15  mm right lower - lobe peripheral pulmonary nodule in actual size ( a , white arrow ) and with magnification ( b , black circle ) . a 24-hour urine specimen for 5-hiaa ( 5-hydroindoleacetic acid ) showed normal result ( 5  mg / day  normal range : 015  mg / day ) . so , the patient underwent wedge resection with video - assisted thoracoscopic surgery as a diagnostic and possible therapeutic procedure . one day after tumor resection the basal morning cortisol and plasma acth levels were significantly decreased to 8  gr / dl ( normal range : 5.528.7  gr / dl ) and 34.41  pg / ml ( normal range : 8.357.8  pg / ml ) , respectively . signs and symptoms of hypercortisolism ( such as hypokalemia , proximal muscle weakness , skin darkening , and weight gain ) recovered gradually . now after 6 months of operation she is well with no complaint , normal k levels , controlled blood sugar , normal suppression of serum cortisol level with low - dose dexamethasone suppression test , and normal chest ct . the causes of endogenous acth - dependent cushing 's syndrome are cushing 's disease ( pituitary adenoma ) in about 80% to 85% of cases and the remaining 15% to 20% are due to ectopic secretion by nonpituitary tumors . hypokalemia is much more prevalent in cushing 's syndrome due to ectopic acth secretion than other causes of cushing 's syndrome . typically , patients with ectopic acth tumors have high acth levels and cortisol levels fail to suppression with high doses of dexamethasone . however , 20% to 40% of patients with ectopic acth demonstrates cortisol suppression on high - dose dexamethasone . so after normal result of pituitary mri , we evaluate the patient for ectopic source of acth . axial imaging with thin - cut multislice ct of chest and abdomen or mri of the thorax has the highest detection rate for ectopic acth syndrome . the most common causes of ectopic acth are small - cell carcinoma of the lung and bronchial carcinoid tumors . bronochpulmonary carcinoid tumors are subclassified by the world health organization into typical and atypical carcinoids , according to histopathologic criteria : typical carcinoids with slow growth that include 90% of lung carcinoids and atypical types grow a little faster with more likely spread to other organs and commonly located in periphery . small peripheral bronchial carcinoids can be missed easily on chest ct scans due to poor inspiratory effort , abdominal fat , and basal atelectasis . so using a modern multidetector high - resolution ct with 2.5  mm slices from lung apex to iliac crests is recommended . with advances in medical and surgical treatments , survival rates for patients with ectopic acth patients with bronchial carcinoids have the best prognosis and considered as a low to moderate grade malignancy . in conclusion , the differential diagnosis of acth - dependent cushing 's syndrome often presents challenging in endocrinology and requires careful clinical , biochemical , radiological , and pathological investigations . carciniod tumors as a treatable cause of hypokalemia and ectopic acth syndrome presented as a small peripheral solitary pulmonary nodule should not be missed in clinical practice .
abstractbackground : differential diagnosis of hypokalemia and adrenocorticotropic hormone ( acth)-dependent cushing 's syndrome often presents challenging in endocrinology and requires careful clinical , biochemical , radiological , and pathological investigations . 
 hypokalemia is a common abnormality and systematic approach is required to avoid delays in diagnosis of important underlying causes.case summary : a 49-year - old woman presented with moderate hypokalemia . 
 further evaluation showed hypercortisolism due to ectopic acth secretion.chest computed tomography ( ct ) revealed a peripheral solitary pulmonary nodule . 
 excision biopsy of the nodule showed carcinoid tumor . 
 after excision biopsy , all of the patient 's symptoms improved and electrolytes and acth levels also became normal.conclusion:carciniod tumors should be considered as a differential diagnosis in patients presenting with hypokalemia and ectopic acth syndrome . 
 carcinoid tumor often present as solitary pulmonary nodule and excision biopsy can be curative .