Last updated on: June 25th, 2021

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A 54-year-old man presents with watery diarrhea for two months. The diarrhea is mainly nocturnal; it is not related to food intake and does not stop even after prolonged fasting. The stools are odorless, without blood or mucus. There is no history of significant weight loss. He was in good health before this. He saw several primary care practitioners over the past one month. He was prescribed a course of loperamide and then a course of metronidazole. However, his symptoms remained unchanged. He is not on any medications right now, including over-the-counter drugs or herbal medications. There is no history of laxative abuse. His medical, surgical, and family histories are unremarkable. He has no known allergies. There is no history of recent foreign or interstate travel.

At presentation, he is mildly dehydrated. His vital parameters are stable. A full blood count, renal profile and liver profile are normal. A electrolyte assay is significant for serum potassium of 2.6 mEq/L (normal: 3.5-5.5) and serum calcium of 7.5 mg/dL (normal: 8.5-10.5). An arterial blood gas assay shows non-anion gap metabolic acidosis with a pH of 7.15 and serum bicarbonate of 6.4 mmol/L. He is then stabilized and resuscitated. Further investigation reveals a serum TSH of 1.5 mIU/L (normal: 0.4-4); a fecal osmotic gap of 15 mmol/L (normal: 50-100); and no leukocytes, bacteria, viruses, fungi, or parasites on stool analysis.

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