Carcinoid syndrome is a rare condition that develops when certain uncommon tumors release powerful chemicals into the bloodstream, creating a collection of symptoms that can significantly affect daily life.

Understanding Carcinoid Syndrome

Carcinoid syndrome happens when a rare type of cancerous growth called a carcinoid tumor secretes unusual amounts of chemicals into your body. These tumors belong to a broader category known as neuroendocrine tumors, or NETs, which develop in specialized cells that are part of your neuroendocrine system. These cells normally produce and release hormones that help regulate various body functions, but when they become tumors, they can flood your system with excessive amounts of these substances.^[1]

The syndrome itself represents a complication of these tumors rather than the tumor alone. When neuroendocrine cells begin to divide and multiply rapidly, they form tumors that release unusually large amounts of serotonin and other substances into the bloodstream. Serotonin is a chemical messenger that affects many processes throughout your body, including digestion, mood, and blood flow. Under normal circumstances, your liver would break down these substances before they could cause problems. However, when carcinoid tumors spread to the liver or grow large enough, the liver cannot keep up with processing all these chemicals, allowing them to circulate freely and trigger the various symptoms of carcinoid syndrome.^[2]

Sometimes people discover they have carcinoid syndrome before they even know they have a neuroendocrine tumor. The appearance of symptoms can be the first sign that prompts doctors to look for these tumors. Most carcinoid tumors develop in the gastrointestinal tract, particularly in areas like the small intestine, appendix, and stomach, or in the lungs. The syndrome typically occurs when these tumors are already advanced, meaning they have grown considerably or spread to other parts of the body.^[3]

How Common Is Carcinoid Syndrome

Carcinoid syndrome is quite uncommon. Neuroendocrine tumors themselves affect roughly 5 in every 100,000 people in the United States, making them relatively rare to begin with. Among those who do have neuroendocrine tumors, only about 1 in 5 people develop carcinoid syndrome. This means the syndrome affects an even smaller portion of the population.^[2]

The incidence of neuroendocrine tumors has been increasing over recent years, though this rise is likely due to better diagnostic tools and greater awareness among healthcare providers rather than an actual increase in disease occurrence. Studies estimate the annual incidence of neuroendocrine tumors to be approximately 6.98 cases per 100,000 individuals. Despite these rising detection rates, carcinoid syndrome remains a rare condition that many general practitioners may only encounter a few times throughout their careers.^[3]

Carcinoid syndrome is most common in people who have neuroendocrine tumors in their small bowel that have spread to the liver. It can also occur in people with lung and pancreatic NETs, though this is less frequent. The syndrome typically develops only when tumors have metastasized, meaning they have spread beyond their original location, particularly to the liver where the normal breakdown of hormones becomes overwhelmed.^[6]

What Causes Carcinoid Syndrome

The root cause of carcinoid syndrome is the presence of neuroendocrine tumors that have metastasized to the liver. The majority of these tumors originate from the midgut, which includes the appendix and small intestine. When tumors remain confined to their original location without spreading to the liver, they typically do not produce carcinoid syndrome because the liver efficiently metabolizes the biogenic amines that the primary tumor produces.^[3]

Once neuroendocrine tumors spread to the liver, however, this natural filtering system becomes compromised. The liver cannot process all the excessive hormones and other bioactive substances being released by the tumors. As a result, these chemicals enter the general circulation throughout your body, triggering the wide range of symptoms associated with carcinoid syndrome. Less commonly, tumors arising from the foregut, which includes the lungs, stomach, and duodenum, or from the thorax can also result in carcinoid syndrome.^[3]

The underlying mechanism involves specialized cells in your neuroendocrine system that have transformed into cancer cells. These cells retain their ability to produce hormones and other bioactive substances, but they do so in an uncontrolled manner. The most commonly secreted substance is serotonin, though carcinoid tumors can also release other chemicals including histamine, bradykinin, and various peptides. Each of these substances contributes differently to the syndrome’s symptoms.^[1]

Risk Factors for Developing Carcinoid Syndrome

The primary risk factor for developing carcinoid syndrome is having a neuroendocrine tumor, particularly one located in the gastrointestinal tract or lungs. However, not everyone with these tumors will develop the syndrome. The risk increases significantly when the tumor has spread to the liver, as this is when the body’s ability to break down excess hormones becomes overwhelmed.^[2]

Among people with neuroendocrine tumors, those with tumors in the midgut region are at highest risk for developing carcinoid syndrome. Midgut tumors, which arise from the small intestine, appendix, and parts of the colon, are more likely to cause the syndrome compared to tumors in other locations. The syndrome is particularly common when these midgut tumors have metastasized to the liver.^[3]

While the exact causes of why neuroendocrine tumors develop in the first place are not fully understood, having these tumors represents the main pathway to developing carcinoid syndrome. The larger the tumor burden and the more extensive the liver involvement, the higher the likelihood that someone will experience carcinoid syndrome symptoms. Some people may have small, stable neuroendocrine tumors for years without ever developing the syndrome, while others may experience symptoms relatively quickly after their tumors begin to grow or spread.^[6]

Recognizing the Symptoms

The symptoms of carcinoid syndrome vary depending on which chemicals the tumor releases into the bloodstream, but several hallmark symptoms appear frequently. The most common symptom is skin flushing, which affects roughly 85 out of every 100 people with carcinoid syndrome. During these episodes, the skin on your face and upper chest feels hot and changes color, ranging from pink to purple. These flushing episodes may last anywhere from a few minutes to several hours or even longer. They can happen without any obvious trigger, though certain factors like stress, physical exercise, or drinking alcohol frequently provoke them.^[1]

Diarrhea is another prominent symptom, affecting about 80 out of every 100 people with carcinoid syndrome. This isn’t ordinary diarrhea but often involves frequent, watery stools that may be accompanied by abdominal cramping. The diarrhea can be explosive and severe, with some people experiencing bowel movements more than 30 times per day. The stools may also be fatty and have a particularly unpleasant smell due to malabsorption issues.^[2][6]

Breathing difficulties represent another challenging symptom. People with carcinoid syndrome may experience asthma-like symptoms including wheezing and shortness of breath, particularly during flushing episodes. These breathing problems can be frightening and may occur suddenly. Some individuals also notice their heart racing or experiencing palpitations, where they become acutely aware of their heartbeat or feel like their heart is beating irregularly or too fast.^[1]

Additional symptoms can include swelling in the feet and legs, which may indicate developing heart problems. Some people experience yellowing of the skin and whites of the eyes, a condition called jaundice. Dizziness or feeling faint can occur due to low blood pressure. Facial skin lesions may appear as purplish areas of spider-like veins on the nose and upper lip. Some individuals report decreased interest in sex or erectile dysfunction. The range of symptoms can make daily activities challenging and significantly impact quality of life.^[2]

Preventing Carcinoid Syndrome

Because carcinoid syndrome develops as a complication of neuroendocrine tumors, true prevention would involve preventing these tumors from forming or spreading in the first place. Unfortunately, the causes of neuroendocrine tumors are not well understood, and there are no established methods to prevent their development. However, people who have been diagnosed with neuroendocrine tumors can take steps to prevent or minimize carcinoid syndrome symptoms.^[13]

For those already living with carcinoid syndrome, avoiding certain triggers can help prevent symptom flare-ups. Dietary modifications play a significant role in symptom management. Foods and beverages that are high in amines can trigger or worsen flushing episodes. This means avoiding aged or blue cheeses, smoked meats like salami or sausage, pickled fish such as herring, and fermented foods like miso or sauerkraut. Chocolate should also be limited. Additionally, foods high in serotonin, including nuts, bananas, and raw tomatoes, may aggravate symptoms.^[13]

Alcoholic beverages, particularly red wine and beer, commonly trigger flushing and other symptoms, so abstaining from alcohol can provide relief. Caffeinated drinks can also provoke symptoms in some people. Rather than eating three large meals daily, consuming four to six smaller, nutritious meals throughout the day can help minimize digestive symptoms. Staying well-hydrated is particularly important for those experiencing frequent diarrhea, as dehydration can develop quickly.^[5][13]

Stress management represents another important preventive strategy. High stress levels can trigger severe episodes of flushing and other symptoms. Practicing relaxation techniques, engaging in breathing exercises, and avoiding unnecessarily stressful situations when possible can help reduce symptom frequency and severity. Similarly, avoiding strenuous physical activity can prevent symptom flare-ups, though gentle exercise like short walks remains beneficial and should not be completely eliminated.^[13]

How Carcinoid Syndrome Affects the Body

Carcinoid syndrome creates widespread changes in normal body functions through the excessive release of bioactive substances, particularly serotonin, into the bloodstream. These substances act as powerful chemical messengers that affect multiple organ systems simultaneously. Understanding what happens in the body helps explain why the symptoms are so varied and can be so disruptive to daily life.^[3]

When carcinoid tumors release large amounts of serotonin, this chemical acts on blood vessels throughout the body, causing them to dilate or widen suddenly. This vascular response creates the characteristic flushing seen in carcinoid syndrome. The redness and warmth felt in the face, neck, and upper chest result from increased blood flow to these areas. The blood vessels in the skin expand rapidly, bringing more blood to the surface and creating the visible color change. This same mechanism can trigger drops in blood pressure, leading to dizziness or lightheadedness.^[1]

The gastrointestinal effects stem from serotonin’s powerful influence on intestinal function. Normally, serotonin helps regulate the movement of the digestive tract, but excessive amounts speed up intestinal motility dramatically. The intestines contract more frequently and forcefully, pushing contents through too quickly for proper absorption of water and nutrients. This results in the watery diarrhea characteristic of the syndrome. The rapid transit also prevents adequate fat absorption, leading to fatty, foul-smelling stools and potential nutritional deficiencies over time.^[6]

Breathing difficulties occur when bioactive substances cause the airways to narrow, similar to what happens in asthma. The smooth muscle surrounding the bronchial tubes contracts in response to these chemicals, making it harder to move air in and out of the lungs. This bronchospasm creates the wheezing and shortness of breath that often accompany flushing episodes. The simultaneous occurrence suggests these symptoms share common chemical triggers released by the tumor.^[1]

Perhaps the most serious pathophysiological change involves the heart. Prolonged exposure to excessive serotonin and other substances can damage heart structures, leading to a condition called carcinoid heart disease. These chemicals cause scarring and thickening of the heart valves and the inner lining of heart chambers. The right side of the heart is typically more affected because blood from the liver, where tumors often metastasize, flows directly to the right heart before passing through the lungs. This valve damage can eventually lead to heart failure if left untreated.^[2]

Carcinoid syndrome can also affect mental health and cognitive function through its impact on tryptophan metabolism. Tryptophan is an essential amino acid that the body uses to produce both serotonin and niacin. When tumors consume large amounts of tryptophan to make excessive serotonin, less remains available for niacin production. Niacin deficiency can contribute to depression, while the altered serotonin metabolism throughout the body may disrupt normal brain chemistry, affecting mood, behavior, and sleep patterns.^[2]

Another complication involves the mesentery, a fold of tissue that anchors the intestines to the abdominal wall. Neuroendocrine tumors can trigger inflammation and scarring of this tissue, a condition called mesenteric fibrosis. This scarring can eventually restrict intestinal movement and blood supply, potentially causing pain and bowel obstruction. The fibrotic changes develop gradually but represent another way the syndrome affects normal tissue structure and function.^[2]