The short answer
Neuroendocrine tumor symptoms depend on where the tumor is and whether it releases hormones. Non-functioning tumors often cause no symptoms until they grow large, while functioning tumors cause symptoms tied to the hormone released. Because symptoms overlap with common conditions, diagnosis can take years.
Neuroendocrine tumors (NETs) can be functioning (releasing hormones that cause symptoms) or non-functioning (causing symptoms only as they grow or press on nearby structures).
Non-functioning NETs are often found by chance, or once they are large enough to cause pain, bleeding, or a blockage.
Functioning NETs cause symptoms tied to the specific hormone released — for example, flushing and diarrhea, or low blood sugar.
Because these symptoms overlap with common conditions like irritable bowel syndrome or menopause, it can take years to reach a NET diagnosis.
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The full explanation.
The simple version
Neuroendocrine tumors (NETs) start in neuroendocrine cells, which make hormones and are found throughout the body — most often in the digestive tract, pancreas, or lungs. Symptoms depend on two things: where the tumor is, and whether it is "functioning," meaning it releases hormones that cause noticeable effects.
Non-functioning tumors
A non-functioning NET does not release hormones that cause symptoms on their own. It can grow quietly for a long time. When symptoms eventually appear, they usually come from the tumor's size or location rather than from hormones — for example:
- Abdominal pain or bloating
- A blockage in the bowel
- Bleeding, which can show up as blood in the stool
- A cough or shortness of breath, if the tumor is in the lung
Functioning tumors
A functioning NET releases hormones into the bloodstream, and the symptoms depend on which hormone it is:
- Tumors that release serotonin and related substances can cause carcinoid syndrome — flushing (redness and warmth in the face and neck), diarrhea, and wheezing or trouble breathing.
- Tumors that release excess insulin can cause low blood sugar, with shakiness, sweating, or confusion.
- Other, rarer functioning tumors can cause stomach ulcers and acid reflux, high blood sugar, or skin rashes, depending on the hormone involved.
Why diagnosis is often delayed
NET symptoms overlap heavily with common, non-cancerous conditions — irritable bowel syndrome, food intolerances, anxiety, asthma, or menopause, among others. Because NETs are also rare and often slow-growing, it is common for years to pass, and for other explanations to be tried first, before a NET is finally diagnosed. If symptoms like flushing, diarrhea, and wheezing occur together and do not fit neatly into another diagnosis, it is reasonable to ask specifically about a neuroendocrine tumor.
When symptoms are an emergency
A carcinoid crisis is a rare but life-threatening emergency: sudden, severe flushing together with a dangerous rise or fall in blood pressure, a racing heart, and trouble breathing. It can happen on its own or be triggered by stress, anesthesia, or a medical procedure. If this happens, call 911 or go to the nearest emergency department immediately.
Sources
- National Cancer Institute, Gastrointestinal Neuroendocrine Tumors Treatment (PDQ) – Patient Version, accessed August 3, 2026.
- National Cancer Institute, Pancreatic Neuroendocrine Tumors (Islet Cell Tumors) Treatment (PDQ) – Patient Version, accessed August 3, 2026.
- National Organization for Rare Disorders (NORD), Carcinoid Syndrome, accessed August 3, 2026.
Words to know
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Common questions
Why don't non-functioning neuroendocrine tumors cause early symptoms?
A non-functioning tumor does not release hormones that cause noticeable effects, so it can grow for a long time before it causes symptoms. When symptoms do appear, they are usually caused by the tumor's size or location — for example, pain, a blockage in the bowel, or bleeding — rather than by hormones.
What symptoms does a functioning tumor cause?
This depends on which hormone is released. A tumor that releases serotonin and related substances can cause flushing, diarrhea, and wheezing (carcinoid syndrome). A tumor that releases too much insulin can cause low blood sugar, with shakiness, sweating, or confusion. Others can cause stomach ulcers, high blood sugar, or skin rashes.
Why does it take so long for many people to be diagnosed?
NET symptoms — such as diarrhea, flushing, abdominal pain, or fatigue — closely resemble far more common conditions like irritable bowel syndrome, food intolerances, anxiety, or menopause. Because NETs are also rare and slow-growing, it is common for years to pass, and for several other explanations to be tried, before a NET is diagnosed.
What is carcinoid syndrome?
Carcinoid syndrome is a group of symptoms — usually flushing, diarrhea, and wheezing — caused by hormones a NET releases into the bloodstream, most often once the tumor has spread to the liver. It typically means the NET is a functioning tumor whose hormones are no longer being fully broken down by the liver.
When is a NET-related symptom an emergency?
A carcinoid crisis — sudden, severe flushing along with a dangerous change in blood pressure, a racing heart, and trouble breathing — is a medical emergency. It can happen spontaneously or be triggered by stress, anesthesia, or a procedure. Call 911 or go to the nearest emergency department.
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Last updated: 2026-08-03Next planned review: 2027-08-03
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source verified — This page was created with AI assistance and checked against the sources listed on it. Source checking is not a medical review.
Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
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