I’d never heard of Serotonin until I was diagnosed with Neuroendocrine Cancer in 2010. It is frequently discussed, often with contrasting views from the respondents. One common assumption/question is that it is responsible for many things that can go wrong with Neuroendocrine Cancer patients who have serotonin-producing tumours. “It’s the hormones” is an easy assumption to make or an easy answer to give in response to a complex set of circumstances. It’s difficult to get a definitive answer and the science behind the behaviour of our hormones isn’t really 100% tied down.
You may see serotonin referred to as a ‘neurotransmitter’, a ‘chemical’ and a ‘hormone’ – this is complex but it is my understanding that it can add context in respect the role/location of the serotonin, e.g. chemical and hormone are essentially synonymous and are endocrine related whereas neurotransmitter is concerned with the nervous system (the neuro in neuroendocrine) and the brain (more on this below). Consequently, I’ll keep this as basic as I can (author’s note on completion – it was not easy!).
Serotonin and NETs
One thing which is widely accepted and agreed…… Serotonin is definitely involved in Neuroendocrine Tumours, in particular, those resulting in carcinoid syndrome which can manifest as a number of symptoms including but not limited to flushing and diarrhea. Although serotonin is one of the main ‘hormones’ released in excess by certain NETs (mainly midgut), it is not thought to be the main culprit behind some of the symptoms produced by Carcinoid Syndrome. For example, flushing, the most common symptom (and a cardinal one) is thought to be caused by a number of hormones/peptides – too many to list but the main ones are histamine (particularly foregut), tachykinins (Substance P), bradykinins, prostaglandins …….. and I’m sure serotonin’s in there too! It does, however, appear to be massively guilty in causing carcinoid syndrome diarrhoea, desmoplasia, and carcinoid heart issues.
Where does Serotonin come from?
Serotonin’s technical name is 5-hydroxyltryptamine (5-HT). It is converted from 5-Hydrotryptophan (5-HTP) which is also known as oxitriptan. 5-HTP is a naturally occurring amino acid and chemical precursor as well as a metabolic intermediate in the biosynthesis of serotonin (…..and melatonin) from tryptophan. Tryptophan is interesting as that brings in one of the missing pieces of the jigsaw – food! Tryptophan cannot be manufactured in the body, it must be brought in via diet. There is no serotonin in food, it is only manufactured in the body.
Tryptophan in food enters the body and serotonin is created by a biochemical conversion process which combines tryptophan (essentially a protein) with tryptophan hydroxylase (TPH), a chemical reactor. I suspect other substances might be involved in that process. There are two forms of tryptophan hydroxylase – TPH1 and TPH2, which are encoded on two independent genes. TPH1 is linked to peripheral serotonin while TPH2 is related to brain serotonin.
While serotonin cannot cross the blood-brain barrier, tryptophan can, and once there, almost all of it is converted to serotonin. Unlike, peripheral setotonin where only a small percentage is used to generate serotonin. Just to emphasise that NET dietitians do not say to avoid foods containing tryptophan other than at the time of marker testing (see below and nutrition Blog 4). When you look at the role of tryptophan in the manufacture of brain serotonin, this might have an adverse effect (see Serotonin and the Brain section below).
The introduction of Somatostatin analogues (SSAs) such as Octreotide and Lanreotide, help reduce the secretion of “tumour-derived serotonin” by binding to its receptors on the outside of the cell. If you ever wondered why receptors are important, please check out my blog on this subject (click here). I mentioned tryptophan hydroxylase (TPH) above and that is actually very interesting as this is how Telotristat Ethyl (XERMELO) is able to help with the symptoms of Carcinoid Syndrome diarrhea (not adequately controlled by SSAs) or where patients are unable to be treated by somatostatin analogues for whatever reason. It’s a potent inhibitor of TPH which will disrupt the manufacturing of tumour-derived serotonin. There is also evidence that it can help reduce the effects or halt the growth of the fibrosis leading to carcinoid heart disease. Slight digression but useful to aid/enhance understanding at this point. Read about Telotristat Ethyl here.
Serotonin and the Brain
There is constant discussion and assumption that serotonin-producing tumours are somehow causing depression, anxiety and rage. Not as simple as that, it’s way more complicated and some big assumptions are being made fuelled by ‘patient forum speculation”. Unfortunately badly moderated patient groups allow these assumptions to go unchallenged and hence they turn into myths.
Certain NETs can overproduce serotonin in the gut but the issues concerning depression and anxiety are normally associated with low levels of serotonin in the brain. Serotonin is separately manufactured in the brain (5-10%) and in the gastrointestinal tract (90-95%). The serotonin in the brain must be manufactured in the brain, it cannot be directly reinforced by gut serotonin (peripheral serotonin). It follows that ‘brain serotonin’ and ‘gut serotonin’ are held in separate stores, they are manufactured in those stores and remain in those stores – there is no cross-pollination. This is managed by something called the blood-brain-barrier (BBB). Therefore, excess serotonin from NETs does not infiltrate the brain. As low-level of ‘brain serotonin’ is often linked to depression, it also follows that it’s possible to have high levels of serotonin in the gut but low levels in the brain.
My simple way of thinking about such things as outlined above, is that low levels of tryptophan (a precursor to serotonin) in the brain might be contributing to low levels of serotonin in the brain. This can happen to anyone. To clarify that, I researched the reasons why there could be low serotonin in the brain.
First, let’s dismiss any connection that the type of anti-depressant called Selective serotonin reuptake inhibitors (SSRIs) is involved. It’s thought that SSRIs work by increasing serotonin levels in the brain. Serotonin is a neurotransmitter (a messenger chemical that carries signals between nerve cells in the brain). We already discussed that it’s thought to have a good influence on mood, emotion and sleep. After carrying a message, serotonin is usually reabsorbed by the nerve cells (known as “reuptake”). SSRIs work by blocking (“inhibiting”) reuptake, meaning more serotonin is available to pass further messages between nearby nerve cells. So tryptophan or peripheral serotonin are not really involved in the workings of SSRI and anti-depressant drugs with similar purposes.
It’s also too simplistic to say that depression and related mental health conditions are caused by low serotonin levels (in the brain), but a rise in serotonin levels (in the brain) is said to be able to improve symptoms and make people more responsive to other types of treatment, such as Cognitive Behaviour Therapy (CBT). It’s a big challenge because there isn’t a general consensus from the scientific world about what actually causes depression and the interaction with antidepressant medication.
It’s also too simple to suggest that NET patients get depression and anxiety directly due to all the “hormones” these tumours produce (ergo the assumptions I mentioned above which permeate patient groups unchallenged). Of course hormones are involved in depression and anxiety but these hormones aren’t there just for NET Patients or just there for NET tumours to oversecrete, they have a significant job in the human body (gut and brain) for every single human being on the planet. It’s also known that there’s a lot of interaction and knock on effects between the actions of different hormones and neurotransmitters that can have an effect on levels.
It should also be noted that the precursor to serotonin, tryptophan, does pass through the BBB and it is therefore possible that tryptophan depletion in the gut and brain can lead to less availability of serotonin in the brain. Tryptophan depletion can be caused by dietary restrictions (i.e. lack of tryptophan foods) and also by the effects of certain types of tumours as excess serotonin is made leading to less availability of tryptophan. Both could lead to low serotonin in the brain as less tryptophan arrives in the brain to be converted to serotonin. When NETs make large amounts of serotonin, tryptophan gets used up causing deficiencies (this actually causes other issues including Vitamin B3 deficiency (Niacin) because Niacin is made from tryptophan). When tryptophan stores are low in the brain, it cannot be converted serotonin. which may then cause a deficiency. It follows that foods containing tryptophan remain important for those with consistently elevated serotonin levels, in order to help maintain normal brain serotonin levels. Due to the introduction of somatostatin analogues which help inhibit hormones and more physician awareness of NETs, nowadays these issues are now isolated cases but NET patients with elevated 5HIAA should remain vigilant to the possibilities. Cancer patients without hormone secreting tumours also get anxiety and depression so it’s possible that NET patients can get depression and anxiety in the same way.
Cancer Emotions – the hormonal effect
There are many emotions that come with cancer. For example “Cancer anger” is said to be a normal response to fear, despair and grief – a range of feelings which cancer brings into our lives. This is relevant to ALL cancers It can show as frustration, irritability, emotional withdrawal or aggression. You can feel it whether you have been diagnosed (with any cancer or serious illness) or you are a relative or friend. Cancer anger can happen at any stage of the illness, even years after treatment. I know that many people with cancer suffer from depression, anxiety and anger but they do not all have serotonin-producing tumours. What they do have is a life threatening and/or life changing condition which is bound to have an effect on mind as well as body. Hormones including Serotonin are natural substances found in the body and not just there to service NETs. It follows that the anger some NET patients claim is related to NETs is the same anger described above.
Measuring Serotonin levels
Measuring levels of serotonin is important in both diagnosis and management of certain NETs – although it’s probably sensible to test all potential NET patients during diagnosis when the type of tumour is not yet known. Testing for tumour markers will differ between countries and within countries but the most common standard for testing Serotonin appears to be 5-HIAA (5-hydroxyindoleacetic acid) either via a 24-hour urine test or via a plasma version (mainly used in USA but now creeping into UK). 5-HIAA is the output (metabolite) of 5-HT (Serotonin). Not to be confused with the less reliable ‘serum serotonin’ which is a different test and can be affected by the use of anti-depressants (read here about this).
It’s hard to diagnose a serotonin imbalance in the brain because there’s no way to accurately test the amount. Blood serotonin levels don’t necessarily reflect the levels in your brain. It also has to be remembered that serum serotonin and 5HIAA are not absolute tests, they are simply indicators of a potential problem. I would just add that it is the reuptake of Serotonin in the brain (plus some other stuff) that can cause depression, not the actual level or amount in the brain.
I intentionally did not mention the other common test (Chromogranin A) or other markers as they are measuring different things but you can read about in my Testing for Markers blog.
Serotonin Video with myself and Dr Mike Morse
I made a video in 2019 with Dr Mike Morse sponsored by Lexicon Pharmaceuticals, Inc. It’s all about Carcinoid Syndrome with a slant towards hormones, in particular Serotonin. Entitled “Likely Suspects: How Hormones May Lead to Carcinoid Syndrome – What People Living With Carcinoid Syndrome Need to Know”
You need to register to watch although some of you will already be registered and just need an email to login to see the this webcast. The one I’m featured in is the latest in a series on the subject and I’d like to break the record for views please! Please help me achieve this 💙 I would also love to get your feedback and sincerely hope you will find the time to listen in. Please also find the time to complete the survey at the end. Thanks
Click on the link here: www.CarcinoidWebcast.com
Don’t forget to press the play button and ensure your sound is turned up, particularly on mobile devices.
Let me start by saying that Serotonin Syndrome has absolutely nothing to do with Neuroendocrine Cancer or carcinoid syndrome. But I add it here as an add-on to hopefully clear up confusion (and overlap) between the symptoms of carcinoid syndrome and serotonin syndrome.
Serotonin syndrome can occur when you increase the dose of such a drug or add a new drug to your regimen. Certain illicit drugs and dietary supplements also are associated with serotonin syndrome. Serotonin syndrome is a potentially serious negative drug reaction. If you take different prescribed medications together, you may end up with too much serotonin in your body. The types of medication that could lead to serotonin syndrome include those used to treat depression and migraine headaches, and manage pain. Too much serotonin in the brain can cause a variety of mild to severe symptoms. These symptoms can affect the brain, muscles, and other parts of the body.
You may have symptoms within minutes or hours of taking a new medication or increasing the dose of an existing medication. The symptoms may include:
– muscle spasms
– muscle rigidity
– rapid heartbeat, or tachycardia
– high blood pressure
– overactive reflexes, or hyperreflexia
– dilated pupils
In more severe cases, the symptoms may include:
– irregular heartbeat
Examples of drugs and supplements associated with serotonin syndrome include (but not limited to):
- Antidepressants such as selective serotonin reuptake inhibitors (SSRIs), e.g. Celexa and Zoloft.
- Serotonin and norepinephrine reuptake inhibitors (SNRIs), such as Effexor.
- Tricyclic antidepressants, such as nortriptyline and amitriptyline.
- Monoamine oxidase inhibitors (MAOIs), such as Nardil and Marplan.
- Certain other antidepressants.
- Migraine medications (triptan category), these include, almotriptan (Axert), naratriptan (Amerge), sumatriptan (Imitrex).
- Illegal drugs can also have some effect, these include: LSD, ecstasy (MDMA), cocaine and amphetamines.
- Herbals supplements have been associated including St. Johns wort and gingseng.
- Illicit drugs, including LSD, Ecstasy, cocaine and amphetamines.Read more of this from Mayo Clinic here.
If you have carcinoid syndrome symptoms and elevated serotonin / 5HIAA levels, always double check with your specialists if you are taking or prescribed any of the above just to be sure there is no conflict.
Remember, Serotonin Syndrome has nothing to do with Neuroendocrine Cancer or Carcinoid Syndrome. I include this section because I looked at the statistics of people prescribed SSRIs. An example from USA – the report from the Centers for Disease Control and Prevention’s National Center for Health Statistics found that 11 percent of Americans over the age of 12 takes an antidepressant, with about 14 percent taking the medication for more than 10 years. It follows that many people with NET will be taking SSRIs regardless of whether they had a NET or not.
I did say it was a difficult jigsaw!
Thanks for reading