What are carcinoid tumors?
Carcinoid tumors are a type of neuroendocrine tumor (NET) in specialized hormone-producing cells found throughout the body, particularly in the GI tract, lungs, and appendix.
These cells are akin to tiny responders, releasing hormones and amines to regulate digestion, blood flow, stress signals, etc.
One of their primary messengers is serotonin. In a healthy state, serotonin helps coordinate gut motility, moving food and waste through the digestive tract at the right pace. When the body perceives stress, whether from toxins, infection, or other threats, serotonin output increases to accelerate this process, acting as a protective flush.
In carcinoid tumors, this protective mechanism is stuck in the “on” position. The tumor’s cells continue producing serotonin in large, unregulated amounts, flooding the system.
What are symptoms of carcinoid tumors?
Over time, persistently high serotonin levels affect multiple systems in the body:
- In the skin and blood vessels, serotonin can trigger repeated flushing episodes.
- In the gut, it increases motility, leading to chronic diarrhea and abdominal cramping.
- In the heart, serotonin stimulates fibrous tissue growth on valves, especially those on the right side, which can impair their ability to open and close properly, a condition known as carcinoid heart disease.
- Another consequence is niacin (vitamin B3) deficiency. Serotonin is made from the amino acid tryptophan, which is also required to make niacin. When large amounts of tryptophan are continually diverted toward serotonin production, there’s less available to produce niacin, potentially leading to pellagra, a condition marked by dermatitis, diarrhea, and cognitive changes.
Local tumor effects:
- Abdominal pain or cramping
- Unexplained weight loss
- Bowel obstruction (nausea, vomiting, constipation)
- Persistent cough or wheezing (lung tumors)
- Blood in the stool or rectal bleeding
Hormone-related effects (carcinoid syndrome):
- Sudden flushing of the face and neck
- Chronic diarrhea
- Abdominal cramping
- Wheezing or shortness of breath
- Rapid heartbeat (palpitations)
- Low blood pressure episodes
- Dilated blood vessels on the skin (telangiectasias)
- Fatigue and weakness
Long-term complications:
- Carcinoid heart disease: damage to heart valves, especially on the right side
- Niacin (vitamin B3) deficiency: pellagra (dermatitis, diarrhea, dementia) from tryptophan depletion due to excess serotonin production
But isn’t serotonin the ‘happy hormone’?
It is extremely common to hear serotonin referred to as the “happy hormone” or “happy neurotransmitter.” I think it’s important to realize that while all substances in the body play a unique role, there aren’t “good” or “bad” hormones. We want everything to be in balance. Even though serotonin may impact our mood, it has countless other functions and is frequently used as a marker of stress.
In fact, high serotonin levels are found in several types of cancer, not just neuroendocrine tumors. A cancerous environment is, by nature, under significant stress, so elevated serotonin fits that pattern. The goal is not to eliminate it, but to maintain optimal, physiologic levels.
While serotonin is the best-known, carcinoid tumors can release a whole cocktail of biologically active substances. Depending on their location, they may secrete histamine, prostaglandins, or even ACTH, a hormone that can trigger excess cortisol production and cause Cushing’s syndrome. This explains why symptoms can vary so widely between patients.
“The normal intestine contains about 95% of the serotonin in the body (and the brain normally contains only about 1%), and in the normal person only about 1% of the dietary tryptophan is converted to serotonin. But in an advanced case of carcinoid, 60% of the tryptophan can be turned into serotonin. Especially if the tumor has invaded the liver, the serotonin won’t be destroyed by the liver in the usual way, and will circulate in the bloodstream at high levels, producing symptoms of flushing, sweating (sometimes dark-colored), diarrhea (serotonin stimulates small intestine smooth muscle, but inhibits the large [Bennett & Whitney, 1966]), nausea, anxiety, reduced urination, muscle and joint pains, and, in late stages, very often cardiovascular disease (especially inflammation, fibroma and calcification of the valves in the right side of the heart) and aggressive behavior (Russo, et al., 2004) and psychosis” (Dr. Ray Peat, 2015)
Carcinoid tumors are considered rare, with an estimated 2–5 cases per 100,000 people per year, though better imaging and diagnostics have made them easier to detect.
Many grow slowly and can go unnoticed for years, only causing symptoms once they’ve spread or begun overproducing hormones. The standard approach to treatment is surgical removal when possible.
Part of treatment for carcinoid tumors should involve reducing the harmful systemic effects of the tumor secretions. Addressing these effects can help protect tissues and restore balance.
Additional therapies:
- Aspirin: inhibits cyclooxygenase (COX) enzymes, lowering prostaglandin production. Prostaglandins can promote inflammation, increase vascular permeability, and contribute to tumor-related symptoms.
- Cyproheptadine: a serotonin receptor antagonist that blocks serotonin’s effects on smooth muscle, gut motility, and blood vessels, reducing flushing, diarrhea, and abdominal cramping.
- Progesterone: has anti-catabolic and anti-inflammatory effects, and can blunt excessive cortisol output by competing for glucocorticoid receptors, helping to prevent tissue breakdown and immune suppression.
- Niacinamide (vitamin B3): replenishes stores depleted when tryptophan is diverted toward serotonin production. Niacinamide also supports efficient energy metabolism and can reduce inflammatory signaling.
Carcinoid tumors are interesting because they show us that many disease processes are exaggerated or misdirected versions of protective functions. Restoring balance often means understanding and addressing the original purpose of the response.
The body’s defenses are trying to keep us safe. With carcinoid tumors, hormone release began as a short-term survival tool—to flush out harmful substances, move resources where they’re needed, and protect tissues. But when this process runs nonstop, it drains nutrients, damages organs, and disrupts normal communication between systems.
Understanding that the body isn’t making random mistakes, but rather pushing a survival program past its healthy limits, points us toward correcting the root cause instead of only chasing symptoms.