S2472 A Case of Functional Bowel Disease Misdiagnosed as Carcinoid Syndrome

Abstract

Introduction: Carcinoid tumor is a well-differentiated neuroendocrine tumor (NET) that arises from enterochromaffin cells. NETs produce serotonin which is metabolized to the inactive 5-hydroxyindoleacetic acid (5-HIAA) by the liver and the lungs. Carcinoid syndrome results from the secretion of serotonin into the systemic circulation by a metastatic NET that originates from the midgut. The primary screening method for carcinoid syndrome is 24-hour urinary 5-HIAA (U-5HIAA) which has a 90 percent sensitivity and specificity in diagnosing carcinoid syndrome. This case illustrates a unique scenario of a markedly elevated U-5HIAA not due to NET. Case Description/Methods: A 29-year-old female presented to our clinic with abdominal pain, bloating, diarrhea, and weight loss associated with postprandial facial flushing. Medications included Dextroamphetamine and Fremanezumab. Based on her symptoms, U-5HIAA was ordered and was elevated at 148 mg (normal < 6). Carcinoid Syndrome was suspected based on symptoms and markedly elevated U-5HIAA. Imaging studies were ordered to identify primary or metastatic NET. These included negative CT abdomen/pelvis and a negative Ga68 Dotatate NETSPOT PET scan. Upper endoscopy and Colonoscopy were also negative. The patient expressed difficulty in stopping dextroamphetamine for the urine collection but due to the negative evaluation for NET she finally agreed to hold it for 3 days. Repeat U-5HIAA decreased down to 10.4 mg. Her symptoms were then believed to be secondary to functional bowel disease. The patient’s symptoms improved with mirtazapine and dicyclomine, and she regained the weight. Discussion: This is a unique case with a misleading U-5HIAA. While mildly elevated levels of U-5HIAA can be seen with tryptophan rich foods and certain drugs, including amphetamines, marked elevation, like in our case is more specific and usually seen with the carcinoid syndrome. Our patient had symptoms suspicious of carcinoid syndrome and a U-5HIAA of 148 mg, which is 25 times the upper normal. However, there was no evidence for NET on extensive work up, and the levels rapidly decreased with stopping dextroamphetamine. To our knowledge, this is the highest falsely positive U-5HIAA reported. This case shows the importance of a complete drug history, including over-the-counter medications. When the 5-HIAA is elevated, the test needs to be repeated after discontinuing all potentially interfering medications.