A 54-year-old man presents with watery diarrhea for two months. The diarrhea is mainly nocturnal and unrelated to food intake. It does not abate with fasting. The stools are odorless, and without blood or mucus. There is no history of significant weight loss.
Over the last month, he has sought medical attention from several primary care practitioners. He was, separately, prescribed a course of loperamide and a course of metronidazole, both of which were without effect. He is currently not on any medications.
His medical, surgical, and family histories are unremarkable. He denies any history of allergies, recent travel to tropical areas, food intoxication, laxative abuse, or use of over-the-counter drugs or herbal medications.
He is found to be mildly dehydrated, but with stable vital parameters. A complete blood count, renal functions, and liver profile are all normal. However, an electrolyte assay is significant for hypokalemia (2.6 mEq/L) and hypocalcemia (3.2 mmol/L), while an arterial blood gas assay shows non-anion gap metabolic acidosis (pH: 7.15, bicarbonate level: 6.4 mmol/L).
He is stabilized and resuscitated appropriately. Follow-up investigations show a serum TSH of 1.5 mIU/L (normal: 0.4-4); no leukocytes, bacteria, viruses, fungi, or parasites on stool analysis; and a fecal osmotic gap of 15 mmol/L (normal: 50-100).