Carcinoid Syndrome

Carcinoid syndrome is due to an underlying carcinoid tumour. These tumours may contain and secrete a number of biologically active substances. Immunocytochemical or radioimmunoassay studies show that carcinoids can contain adrenocorticotrophic hormone (ACTH), gastrin, somatostatin, insulin, motilin, growth hormone, gastrin- releasing peptide, serotonin, calcitonin, neurotensin, melanocyte- stimulating hormone-beta, tachykinins (substance P, substance K, neuropeptide K), glucagon, pancreatic polypeptide (PP), vasoactive intestinal peptide (VIP), and prostaglandins. These substances may not be released in sufficient amounts to cause symptoms.

In various studies of patients with carcinoids, elevated serum concentrations of PP occur in 43%, motilin in 14%, and subunits of human chorionic gonadotropin (HCG) in 12%; a slightly elevated level of gastrin was reported in 15%, and none were reported with an elevated VIP or plasma gastrin-releasing peptide level. Even though these gastrointestinal (GI) peptides were present in the serum, it is not apparent that any of these peptides contributed to any clinical symptoms. Foregut carcinoids are more likely to produce various GI peptides than midgut carcinoids.

5-hydroxyindoleacetic acid is the metabolic product excreted in urine from the metabolism of serotonin.

Ectopic ACTH production with Cushing's syndrome is increasingly seen with foregut carcinoids, and in some studies, these tumours were the most common cause of the ectopic ACTH syndrome, accounting for 64% of all patients. Acromegaly due to release of growth hormone-releasing factors can occur with a number of carcinoids.
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