Orientation & Validation: What is Carcinoid Syndrome?

If you are reading this, you may have spent months or even years searching for an explanation for sudden flushing, unexpected diarrhea, or vague abdominal cramping ^[1][2]. Finding out that these symptoms have a name—Carcinoid Syndrome—is often a moment of profound validation. It is important to understand that while the underlying neuroendocrine tumor (NET) is a cancer that requires serious management, Carcinoid Syndrome is a paraneoplastic syndrome, a group of symptoms that occur when a tumor releases specific chemicals into your bloodstream ^[3][4].

What is Carcinoid Syndrome?

The syndrome occurs when certain neuroendocrine tumors (NETs) become “functional,” meaning they actively produce and secrete bioactive substances ^[3]. The primary culprit is usually serotonin, a hormone that normally regulates mood and digestion but, in excess, can cause your body to overreact ^[5][6].

While serotonin is the most common substance involved, these tumors can also release other chemicals like histamine or tachykinins ^[7]. When these substances reach your general circulation, they cause the hallmark “flushing” (redness and warmth in the face and neck) and “secretory diarrhea” (watery bowel movements) associated with the condition ^[8][3].

Why the Diagnostic Journey is Long

It is common for patients to feel frustrated by how long it took to reach this diagnosis. On average, it can take several years from the first symptom to a correct identification ^[1]. This delay happens because the symptoms mimic much more common, less serious conditions:

• Irritable Bowel Syndrome (IBS): The chronic diarrhea and cramping are often mislabeled as IBS for years ^[1].
• Menopause: Flushing is frequently dismissed as “hot flashes” ^[9].
• Food Allergies: Because some foods can trigger symptoms, many patients are first told they have a simple food intolerance ^[2].
• Asthma: If the chemicals cause the airways to tighten, the resulting wheezing may be treated as standard asthma ^[8].

Validation of your experience is a key part of your care. Your symptoms are not “in your head” or “just stress”—they are a physical response to hormones being released by a tumor ^[3].

Who is Affected?

Carcinoid Syndrome does not affect everyone with a neuroendocrine tumor. Its prevalence varies substantially by primary site and tumor biology, but it is estimated to occur in about 20% of the overall NET patient population ^[7]. It is most common in people with “midgut” tumors (those starting in the small intestine, appendix, or cecum) that have metastasized (spread) to the liver ^[7][10].

The reason the liver is so important is that it usually acts as a filter. When a tumor is only in the digestive tract, the chemicals it produces are sent straight to the liver and neutralized before they can reach the rest of your body ^[11]. However, if the tumor has spread to the liver, these substances can leak directly into your general bloodstream, causing the symptoms you feel ^[11]. In rare cases, tumors in the lungs or ovaries can cause the syndrome without spreading to the liver because they bypass this filtering system entirely ^[11][12].

Confirming the Diagnosis

To establish Carcinoid Syndrome, doctors look for clinical symptoms and investigate the underlying NET with biochemical tests and imaging ^[4].

• Biochemical Testing: The most common test is a 24-hour urinary 5-HIAA test, which measures a byproduct of serotonin in your urine ^[13][4]. Some centers now use a blood (plasma) test for 5-HIAA instead ^[13].
• Imaging: Specialized scans like a Ga-68 DOTATATE PET/CT are used to find the location of the tumors ^[14][15].

Note: While a normal 5-HIAA result is common, it does not completely rule out the syndrome, as non-serotonin-mediated or intermittent symptoms can occur. Information regarding complications and emergency “red flag” symptoms can be found on the Complications page.