NETs: Key Facts

*Many people with NETs often don’t have symptoms. Symptoms will also vary depending on the person and the location of the tumor.

The neuroendocrine system is made up of nerve and gland cells called neuroendocrine cells. These cells release hormones into your body that generally regulate the function of different organs.

When neuroendocrine cells change and grow uncontrollably, NETs can develop. NETs may be non-cancerous (benign) or cancerous (malignant).

Benign NETs tend to grow slowly and are confined to a limited area of the body. They may generally be considered non life-threatening if they do not impact or invade other nearby areas of the body.

Malignant NETs often grow faster or more uncontrollably and can affect the surrounding tissues or move from one area of the body to another (metastasize).

If surgery is not possible, there are other treatment options for people with metastatic NETs.

Symptomatic vs asymptomatic tumors

You may hear NETs referred to as being symptomatic or asymptomatic. Symptomatic NETs are NETs that produce too many hormones and substances that cause symptoms in the body. Asymptomatic NETs generally do not cause specific symptoms.

Carcinoid syndrome can occur in people with symptomatic NETs. For example, if a tumor releases a substance called serotonin, it can cause symptoms of diarrhea or facial flushing. However, there are many other types of symptomatic NETs, each with a variety of hormones released.

Meanwhile, asymptomatic NETs may not release substances at all, or they do not release enough to cause symptoms.

What are carcinoid tumors?

Many people still refer to NETs as carcinoid tumors. This is an older term that is sometimes still used as an alternative for either NETs in general, or NETs that do not originate in the pancreas.

In 1907, NETs were first named carcinoid tumors because scientists thought their slow growth meant they were “cancer-like” (benign) tumors rather than truly cancerous (malignant).

But it is now clear that NETs can be malignant and spread from one part of the body to another, like other forms of cancer.

What is carcinoid syndrome?

Carcinoid syndrome is a distinct group of symptoms that some people with NETs may get when tumors in the gastrointestinal (digestive) system spread to other parts of the body — usually the liver. The main symptoms of this condition include diarrhea, flushing of the skin (particularly the face), and wheezing. In later stages, carcinoid syndrome may damage the heart valves, resulting in symptoms of congestive heart failure called carcinoid heart disease.

The symptoms can vary from person to person, and can occur when GI-NETs release an excessive number of hormones such as serotonin, along with a number of other active peptides, into the bloodstream.

Carcinoid syndrome can affect almost 20% of people with NETs. People with carcinoid syndrome may experience these symptoms unexpectedly over time since the hormones may be produced at any time. This can have a major impact on quality of life.

These symptoms are often misdiagnosed as signs of other illnesses, such as irritable bowel syndrome (IBS), Crohn’s disease, or menopause.

Visit the practical tips and tools section for information on the management of carcinoid syndrome symptoms.

Symptoms of carcinoid syndrome

Primary tumor sites Examples of hormones involved Main symptoms
  • Lungs
  • Stomach
  • Bowel
  • Small intestine
  • Serotonin
  • Bradykinins
  • Tachykinins
  • Diarrhea (including nighttime diarrhea)
  • Flushing of the skin (particularly the face)
  • Heart problems (palpitations)
  • Wheezing and breathlessness

Symptoms of carcinoid syndrome may be triggered by emotional stress and anxiety, drinking alcohol, and by certain food types.

Carcinoid syndrome symptoms may be avoided or alleviated by:

  • Avoiding stressful situations or practicing relaxation techniques
  • Reducing the consumption of alcoholic and caffeinated beverages
  • Avoiding foods that contain high levels of the amino acid tryptophan (eg, chocolate, milk, cheese, red meat, fish, poultry) or serotonin (eg, bananas)
  • Exercise

Carcinoid syndrome can result in vitamin deficiencies and regular bouts of diarrhea, which can cause dehydration. A more serious complication, however, is carcinoid crisis.

Carcinoid crisis is the immediate onset of debilitating and potentially life-threatening manifestations of the same symptoms that are associated with carcinoid syndrome. Please make sure you let doctors know that you have NETs so that they can manage you accordingly.


Find out more about NETs

Diagnosis and Testing

Diagnosing NETs

Find out how NETs are diagnosed and the tools doctors may use to help.


Living with NETs

The emotional, physical, and social challenges of living with NETs.

Living with NETs

Advocacy & Support

Living with NETs is not something that you have to do alone. Find help and support.

Get support
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