Measuring serotonin: what a blood or urine test shows

October 21, 2026approx. 14 minutesby mybody®x Editorial & Expert Team

In this article

Can serotonin levels be measured?

Yes, in blood and through a breakdown product in urine. Both samples come from the body outside the brain, and the blood-brain barrier does not let serotonin through (Kanova & Kohout, 2021).

Anyone looking for a serotonin test has a personal reason. Maybe you’ve been feeling low or restless for a while, and you read somewhere that a serotonin deficiency could be behind it. It’s easy to understand wanting a number. A measurement seems more tangible than a feeling, and promises an explanation you can see in black and white.

The answer to whether serotonin can be measured therefore has two parts. The first is technical: serotonin can be measured in the body, in the blood, where it is mostly stored in platelets (Kanova & Kohout, 2021), or indirectly via its breakdown product in urine (MedlinePlus, 2024). The second part is more important. Whether the measured value answers the question you started with depends on where the measured serotonin comes from.

What is serotonin, and where is it produced?

Serotonin is a messenger substance produced in two places in the body: the brainstem and the intestinal lining (Bamalan et al., 2023). Its chemical name is 5-hydroxytryptamine, or 5-HT for short, and in technical terms it is a neurotransmitter, or messenger substance (Bamalan et al., 2023).

According to the textbook chapter in the StatPearls series, serotonin affects, among other things, the following areas (Bamalan et al., 2023):

• behavior
• mood
• memory
• balance in the digestive tract

The chapter also describes exactly where it is produced (Bamalan et al., 2023). In the brain, it’s the raphe nuclei, groups of nerve cells in the brainstem. In the gut, it’s enterochromaffin cells, specific cells in the intestinal lining.

Serotonin comes up in discussions of low mood because of its role in mood. For testing, another point from that same list matters: digestion. According to a review by Kanova and Kohout (2021), the gut produces almost 95% of the body’s serotonin.

So there isn’t just one serotonin level, but two separate stores (Kanova & Kohout, 2021). That’s why anyone referring to a serotonin level should specify which one they mean. A lab can only measure the store present in the sample.

What does a serotonin test in blood or urine show?

In the blood, serotonin is measured, most of which is stored in platelets (Kanova & Kohout, 2021). In the urine, the breakdown product 5-HIAA is measured, which, according to MedlinePlus (2024), shows how much serotonin the body produces overall.

Both tests relate to serotonin, but they look at different things. One measures a stored supply in the blood. The other measures a breakdown product that ends up in the urine after the body has broken down serotonin (MedlinePlus, 2024).

What does a serotonin blood test measure?

Mostly serotonin stored in platelets (Kanova & Kohout, 2021). Platelets take up serotonin from the blood through a transporter in their membrane and store it in small granules (Kanova & Kohout, 2021).

Bader (2020) describes where this serotonin comes from in a review: produced in the gut and other tissues outside the brain, it supplies the platelets in circulation. A serotonin blood test result therefore mostly describes a store that comes from the gut and is held in platelets (Kanova & Kohout, 2021).

This sounds like a technical question, but it has a practical consequence. If you have a serotonin blood test result in front of you, you’re reading a statement about the blood and the platelets. What it reveals about the brain is a separate question, and the answer is brief (Kanova & Kohout, 2021).

How is the 5-HIAA urine test done?

Urine is collected over 24 hours, and certain foods are avoided for the three days beforehand (MedlinePlus, 2024). The test measures 5-HIAA, or 5-hydroxyindoleacetic acid, a serotonin breakdown product (MedlinePlus, 2024).

According to MedlinePlus (2024), the amount of 5-HIAA shows how much serotonin is present in the body. “In the body” means the whole body, not specifically the brain. According to MedlinePlus (2024), the test is done primarily for a specific reason: to detect carcinoids, certain tumors of the digestive tract, and to monitor the disease in people with such a tumor.

The foods that MedlinePlus (2024) says to avoid for three days before collection are everyday foods. They include:

• Plums
• Pineapples
• Bananas
• Eggplants
• Tomatoes
• Avocados
• Walnuts

Anyone who is supposed to take this test should discuss preparation in advance with the practice that ordered it. The list highlights one thing above all: this is not a quick test you can do on the side, but a procedure that requires advance preparation (MedlinePlus, 2024).

The overview compares the measurements relevant to serotonin and dopamine: what each one measures and how the sources interpret the results.

Overview of measurements

What does blood serotonin reveal about the brain?

Blood serotonin levels say little about the brain (Kanova & Kohout, 2021). The reason: serotonin cannot cross the blood-brain barrier, so the brain and the rest of the body have two separate stores (Kanova & Kohout, 2021).

This sentence is the key point. It is based on two review articles that express it in different words but agree on the substance. Kanova and Kohout (2021) write that serotonin cannot cross the blood-brain barrier. Bader (2020) describes the barrier as impermeable to serotonin.

What does the blood-brain barrier mean for serotonin levels?

It separates the serotonin levels in the blood and brain. The blood-brain barrier is the boundary between the blood and the brain, and serotonin cannot cross it (Bader, 2020).

Production is also separate. The brain and gut produce serotonin using two different enzymes, called TPH2 and TPH1 in the scientific literature (Bader, 2020). TPH2 works in the brain, while TPH1 works in the gut and other tissues outside the brain (Bader, 2020). Bader (2020) describes this as a division of the serotonin system established by the blood-brain barrier.

A logistics analogy makes this easier to grasp. A company operates two warehouses in two cities, and no truck travels between them. Counting the boxes in one warehouse tells you the stock in that warehouse, not the other one’s. Serotonin in the blood and serotonin in the brain work in much the same way (Kanova & Kohout, 2021).

Where does the serotonin in the blood come from?

Mostly from the gut (Kanova & Kohout, 2021). According to Kanova and Kohout (2021), the gut is the body’s largest endocrine organ, meaning it produces hormones, and makes almost 95% of all serotonin.

The blood platelet supply is also replenished from the gut (Bader, 2020). Most of what is measured in the blood comes from the gut (Kanova & Kohout, 2021). An abnormal serotonin level in the blood is therefore primarily an indication of this supply outside the brain.

For someone trying to understand their mood, this is sobering news. For someone who already has a test result in hand, it is useful because it puts the number in context. It describes the gut’s supply in the blood, not the brain.

How the gut and brain are connected is a broader question than what a measurement tells us. It has its own article: how the gut and brain are connected. Here, the focus is on the narrower question of what a lab result reflects.

And if stress mainly upsets your stomach, you’ll find a separate discussion in the article on diarrhea under stress.

Dopamine test and neurotransmitter test: What should you make of them?

Dopamine can be measured in blood or urine as a catecholamine (MedlinePlus, 2024). According to MedlinePlus (2024), these tests are intended to help detect certain rare tumors, and levels can change with stress, among other factors.

Alongside the serotonin test, a second measurement is being considered: dopamine. In the laboratory, dopamine belongs to the catecholamines. MedlinePlus (2024) identifies the main types in this group as:

• Dopamine
• Noradrenaline
• Adrenaline

According to MedlinePlus (2024), these substances can be measured in blood or urine. MedlinePlus (2024), the health encyclopedia of the U.S. National Library of Medicine, sums up what the result means in one sentence. In paraphrase, according to MedlinePlus (2024): If catecholamine levels in blood or urine remain higher than normal, this can be a sign of certain rare tumors.

According to MedlinePlus (2024), the tests are used primarily to detect or rule out such tumors. The statement in the quote is cautiously worded: it refers to persistently elevated levels and a possible sign, not a diagnosis (MedlinePlus, 2024).

What changes dopamine test results?

Everyday life. According to MedlinePlus (2024), these factors affect dopamine levels and those of the other two catecholamines:

• Stress
• Strenuous physical activity
• Caffeine
• Smoking
• Alcohol

Diet before the test is another factor. According to MedlinePlus (2024), certain foods, including those containing caffeine and vanilla, should be avoided for two to three days beforehand.

For questions about mood, there are two drawbacks. According to MedlinePlus (2024), the test is intended for rare tumors. And anyone who is tense and wants to get tested for that reason brings stress itself into the picture, a factor that MedlinePlus (2024) says can change the levels. A dopamine test using blood or urine is therefore not a mood measure, but a test with a clearly defined medical purpose.

How do you evaluate an offer for a neurotransmitter test?

With two questions: Which substance is measured, and in what sample? Only the answers show whether the measurement fits your question.

Neurotransmitter is the technical term for messenger substances such as serotonin (Bamalan et al., 2023). A test name that uses only this umbrella term does not yet tell you what is actually measured in the lab. So ask which specific substance is measured and what sample is used, then compare the answer with the overview above.

If it says blood serotonin, the statement about the brain from the previous chapter applies (Kanova & Kohout, 2021). If it says dopamine in blood or urine, what MedlinePlus (2024) describes about its purpose and fluctuating levels applies. If a service promises insights into your mood, ask what that promise is based on.

Testing for serotonin deficiency: What can measurements show?

They show the supply outside the brain: blood serotonin comes mostly from the gut and does not reach the brain (Kanova & Kohout, 2021). According to MedlinePlus (2024), the 5-HIAA urine test is intended primarily for tumors of the digestive tract.

“Serotonin deficiency” sounds like a lab result. Those who search for it are essentially asking about the brain, mood, and motivation. Put the validated measurements side by side, and they answer a different question.

The blood level describes a supply that mostly comes from the gut and is stored in platelets (Kanova & Kohout, 2021). Because the blood-brain barrier does not let serotonin pass through, this supply remains separate from the brain (Bader, 2020). The urine test measures the breakdown product 5-HIAA and, according to MedlinePlus (2024), reflects the amount throughout the body. According to MedlinePlus (2024), it is used primarily to detect carcinoid tumors and monitor their progression.

So the number on such a test result describes a supply in the body outside the brain. That is not bad news, just an honest one. It shifts the focus to the question that really matters to you.

No mybody®x (MYBODY Lab GmbH) test measures neurotransmitters, either serotonin or dopamine.

If you have been feeling persistently low, this should first be evaluated by a doctor. The same applies to restlessness or lack of motivation that does not go away. At your primary care practice, you can describe how you are feeling and discuss which tests make sense in your situation. In an emergency in Germany, call 112.

You can jot down a few points beforehand for the appointment:

• how long you have been feeling this way
• in which situations it gets better or worse
• what you have tried so far
• whether you have a test result showing serotonin or catecholamine levels that you can bring with you

The article Vitamin Deficiency and Mental Health covers vitamins and mood. Hormones are a separate topic. A separate article explains what is known about estrogen and mental health.

Nutrient blood levels answer a different question from that of serotonin. Which levels a nutrient deficiency test measures is explained in a separate article.

Measuring serotonin: What are the limits of our knowledge?

The statements about serotonin are based on academic literature that did not itself measure anything. The information about the tests comes from a US encyclopedia for the general public.

Kanova and Kohout (2021) wrote a narrative review, that is, a synthesis of existing research without any original measurements, focusing on intensive care medicine. The estimate that almost 95% of serotonin comes from the gut also comes from this review (Kanova & Kohout, 2021).

In his review, Bader (2020) describes a research approach based on animal studies. Only two basic principles are drawn from it here: the brain and gut produce serotonin using different enzymes, and the blood-brain barrier does not let serotonin through (Bader, 2020).

The textbook chapter by Bamalan et al. (2023) covers the two sites where serotonin is produced. For the distribution of serotonin, this text draws on the review by Kanova and Kohout (2021).

MedlinePlus is an encyclopedia for the general public from the U.S. National Library of Medicine (MedlinePlus, 2024). It describes tests from a US perspective. For your preparation, follow the instructions from the healthcare provider who orders the test.

There is also a limitation to the measurement itself. Stress and caffeine can alter catecholamine levels (MedlinePlus, 2024). So a single result also depends on the day the sample was taken. And any blood test for serotonin measures it on the body's side of the blood-brain barrier (Kanova & Kohout, 2021).

Measuring serotonin: What you can do now

Before ordering a serotonin test, clarify what question the result is meant to answer. The blood level reflects the supply outside the brain (Kanova & Kohout, 2021), while a urine test for 5-HIAA is used primarily to look for certain tumors, according to MedlinePlus (2024). If your concern is mood and motivation, speak openly with your healthcare provider about persistent low mood and bring your notes. A measurement may feel more tangible than a feeling, but when it comes to serotonin, talking it through will get you further.

Frequently asked questions

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Sources

1. Kanova M., Kohout P.: Serotonin – Its Synthesis and Roles in the Healthy and the Critically Ill, International Journal of Molecular Sciences (2021) – europepmc.org
2. Bader M.: Inhibition of serotonin synthesis, Pharmacology & Therapeutics (2020) – europepmc.org; Bamalan O. A. et al.: Physiology, Serotonin, StatPearls (2023) – europepmc.org
3. U.S. National Library of Medicine, MedlinePlus: 5-HIAA urine test (2024) – medlineplus.gov
4. U.S. National Library of Medicine, MedlinePlus: Catecholamine Tests (2024) – medlineplus.gov

The information on serotonin originating in the gut, its storage in blood platelets, and its separation by the blood-brain barrier is based on [1] and [2], while the information on where it is produced is based on [2]. The information on the 5-HIAA urine test is based on [3], and that on dopamine and catecholamines on [4]. English sources have been translated in meaning; the quotation in the dopamine chapter is given in the original English. All sources checked on 21.10.2026.