Serotonin is a neurotransmitter — a chemical that relays messages between nerve cells — that plays an important role in regulating mood, sleep cycles, appetite, temperature and digestion. A serotonin blood test can show whether your serotonin level is in the normal range 1. **

Find out why — and how — serotonin levels are tested, along with insight into the relationship between serotonin levels and depression 2.**

What to Expect From a Serotonin Level Test

For a serotonin level test, a blood sample is obtained by inserting a needle into a vein in the arm 1.

** You will need a doctor's order before a lab can do the test. No fasting or other preparation is required before the test.

According to the National Institutes of Health, a normal serotonin range is 50 to 200 ng/mL (0.28 to 1.14 µmol/L) 1. ** Different labs may use slightly different ranges, so discuss your results with your health care provider.

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A serotonin level test is typically ordered to check whether the level of serotonin in your blood is abnormally high 1. Excess serotonin can be a sign of a carcinoid tumor, a class of slow-growing tumors of the small intestine, colon, appendix and bronchial tubes in the lungs 14. Your health care provider may order a serotonin blood test if you have any symptoms of carcinoid syndrome, which occurs when you have this type of tumor, including 14:

• flushing of the face and neck
• diarrhea
• nausea
• vomiting
• rapid heart rate
• wheezing, coughing or difficulty breathing

If you have higher than normal levels of serotonin and carcinoid syndrome symptoms, your doctor may order an imaging scan to locate a potential carcinoid tumor 4.

A high serotonin level and carcinoid syndrome symptoms doesn't necessarily mean you have a tumor 4. If a growth is found, a biopsy must be done to determine if it is a carcinoid tumor.

Is There a Connection Between Serotonin Deficiency and Depression?

**For decades, a popular theory was that depression was caused by a depletion of the monoamine neurotransmitters serotonin, norepineprhine and dopamine in the brain.

** This "monoamine hypothesis" made sense because most antidepressant drugs affect levels of these neurotransmitters. For example, selective serotonin reuptake inhibitors (SSRIs), the most commonly prescribed class of antidepressants, make more serotonin available in the brain synapses by preventing it from being reabsorbed by neurons.

However, the theory continues to be hotly debated. These days, most experts agree that the relationship between serotonin and depression is more complicated.

"Yes, neurotransmitters like serotonin, norepinephrine [and] dopamine are involved in mood disorders, depression, mania [and] psychosis, but we know it’s far more complex than simply too little serotonin," explains Philip R. Muskin, MD, chair of the scientific program committee of the American Psychiatric Association and professor of psychiatry at Columbia University Irving Medical Center 2. ** "It’s not that neurotransmitters aren’t involved — they are — but it’s not so cut and dry."

One key limitation to researching the link is that it's not possible to directly measure serotonin levels in the human brain. And since serotonin doesn't cross the blood-brain barrier, blood serotonin levels doesn't necessarily reflect the levels in the brain 1. ** "The only way to get a measure of serotonin in your brain is to get a spinal tap," Dr. Muskin explains. "There are metabolites of serotonin in our spinal fluid that will give us a sense of how much serotonin is in the brain."

Some studies have found that low serotonin as indicated by spinal fluid is associated with suicidality. However, that's just a correlation — it's not clear which came first. It could be that a low serotonin level causes depression; alternately, depression may leads to a low serotonin level.

Read more: 8 Signs of Depression You Shouldn't Ignore

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"Serotonin in your blood essentially has no meaning" when it comes to revealing anything about serotonin activity in the brain, says Dr. Muskin 1. Although a few studies have demonstrated a link between low serotonin levels in the blood and low mood, there isn't any evidence that blood serotonin levels are consistently lower in patients with depression or other psychological disorders 12. **

Consequently, a lower than normal serotonin blood test is not considered diagnostic for depression or any other condition 1. "We make diagnoses of depression clinically — by talking to someone," Dr. Muskin says. If you are experiencing the symptoms of depression, your health care provider may recommend treatment, regardless of your blood serotonin levels 1.

You may benefit from taking an antidepressant medication, like an SSRI. There are also lifestyle changes that may help 2.

** Studies have shown that exercise improves symptoms of depression in some people with mild to moderate depression 67.

** Exposure to bright light (either natural sunlight or light therapy lamps) may also have an antidepressant effect. While there's some indirect evidence that these interventions affect serotonin, more research is needed to determine the exact mechanism(s) by which they work.

Read more: The Effects of Exercise on Serotonin Levels

Muskin says you should be skeptical of any claims that you should "take certain foodstuffs or herbal preparations" to boost your serotonin level. Some researchers have suggested that supplementation with tryptophan, the amino acid that your body needs to make serotonin, may help increase serotonin.

Read more: List of Foods With Serotonin

However, contrary to popular belief, simply eating foods rich in tryptophan, like poultry, dairy, and nuts, does not seem to have an effect on levels of serotonin in the brain.

"There’s a lot of tryptophan in our diets. The supplementation from food really is kind of worthless," Muskin says.