Metastatic carcinoid tumors (MCTs) are slow-growing tumors that usually start in the gastrointestinal system. However, they can appear in places other than the digestive tract. Eventually, they will metastasize or spread to other locations in the body.

Because MCTs are so slow to grow, they often don’t cause symptoms until they’ve grown significantly. Sometimes they’re discovered only by accident during a surgical procedure or imaging study.

For some people, an MCT can cause symptoms that are known as carcinoid syndrome. These symptoms can range from mildly bothersome to severe. Sometimes they can resemble other conditions, such as irritable bowel syndrome. The result might be a misdiagnosis, or even months of testing and treatment before identifying the condition.

Here, learn about how and why MCT causes the symptoms it does, as well as other conditions the symptoms may resemble.

What are the major symptoms of MCTs?

When you have an MCT, the tumor can give off many substances. If the tumor is in your digestive tract, the substances usually enter your liver and are destroyed before they can cause any symptoms. This is why carcinoid tumors in the digestive tract don’t usually cause symptoms until the tumor spreads to another location, such as the liver.

However, carcinoid tumors can grow in other areas of your body. You’ll likely experience symptoms earlier if the MCT is somewhere other than your digestive tract.

The most common substance that MCTs secrete is serotonin. While your body naturally has a certain amount of serotonin present, extra serotonin from the tumor causes many of the symptoms associated with MCTs. Examples of these effects include:

  • Increased intestinal movement: Serotonin irritates and excites the muscles of your intestines. The intestines start to move more quickly than normal, which causes diarrhea. Over time, the constant diarrhea can lead to dehydration, weight loss, and malnutrition.
  • Bronchoconstriction: Inside your lungs are airways that look like tree branches. The extra serotonin makes these airways constrict, or get tighter. As a result, you may start to wheeze and have difficulty breathing.
  • Platelet aggregation: Also known as blood clotting. Serotonin can cause the platelets in your blood to stick together, which can create a blood clot.
  • Blood vessel constriction or enlargement: Blood vessels can get wider or narrower. This can cause changes in blood pressure and heart rate. It also causes flushing, or a feeling of intense heat and skin redness.

Doctors call this collection of symptoms carcinoid syndrome. According to a 2006 study, only an estimated 10 percent of those with a carcinoid tumor have symptoms of carcinoid syndrome. Sometimes the symptoms a person experiences depends on where the tumor is located, such as in their small intestine, appendix, rectum, lungs, or pancreas.

When a person has had MCT for a while, they may also start to develop heart problems. Examples include valve problems on the right side of their heart and scarring of heart tissue. According to a 2004 study, anywhere from 20 to 70 percent of people with MCTs also have a heart condition called carcinoid heart disease (CHD).

What are some factors that can worsen MCT symptoms?

Some people notice MCT symptoms more after they’ve exercised. In addition to activity, eating certain foods and drinks can bring on MCT symptoms. Examples include foods that contain tyramine, such as:

  • beers on tap
  • blue cheese
  • chocolate
  • cured meats
  • fava beans
  • products made from soy, such as miso soup

The ethanol compound in red wine may also increase risk for MCT symptoms.

Stress and anxiety can worsen symptoms of MCTs as well.

However, carcinoid syndrome can occur even when a person hasn’t eaten anything that triggers MCT or doesn’t feel stressed.

Stress to the body from surgery can also lead to a carcinoid crisis. For this reason, those with a known MCT usually receive special medicines before surgery.

What conditions may have symptoms like those of MCTs?

Because an MCT rarely cause symptoms until it has progressed, doctors don’t often identify it in its earlier stages. Several medical conditions can cause symptoms that are similar to those of an MCT. A doctor will likely rule these out before making a final diagnosis of MCT.

One of the hallmark symptoms is flushing. Other conditions that could cause flushing include:

  • medications or an excess of certain foods that can cause flushing, such as those containing niacin
  • mastocytosis, a condition that causes too many mast cells (immune system cells) to build up in the body
  • medullary carcinomas of the thyroid gland
  • menopause
  • pheochromocytoma, a noncancerous tumor that develops in the adrenal glands above the kidneys

Wheezing is another common symptom associated with MCTs. Examples of other conditions that can cause wheezing similar to an MCT include:

  • anaphylaxis, a strong and potentially life-threatening allergic reaction
  • asthma
  • presence of a foreign body, or something a person has swallowed that makes it difficult to breathe
  • pulmonary edema, excess fluid in the lungs that makes breathing difficult

The diarrhea that can occur with MCTs may also resemble other medical conditions, including:

  • gastroenteritis, or upset and irritated stomach
  • infectious colitis, such as food poisoning
  • inflammatory bowel disease
  • overuse of laxatives

Heart valve abnormalities are another symptom associated with MCTs. Conditions that can cause heart valve abnormalities include:

  • dilated cardiomyopathy, in which the heart becomes enlarged and doesn’t pump properly
  • ischemic heart disease, or narrowing of the heart arteries, resulting in dysfunction of the heart’s papillary muscle
  • rheumatic heart disease, in which heart valves are damaged as a result of rheumatic fever
  • subacute bacterial endocarditis, an infection of the heart

If you have symptoms you suspect could be a carcinoid tumor or you have a family history of carcinoid tumors, talk to your doctor about your risk. Doing so could help to quickly reach or rule out a diagnosis.