Serotonin: What It Is, Functions And Normal Ranges

“Serotonin is a neurotransmitter that plays multiple roles in the central nervous system, as well as in other organ systems, such as the gut, throughout the body,” explains Chris Winter, M.D., a neurologist based in Charlottesville, Virginia.

Also known as 5-hydroxytryptamine (5-HT), serotonin is a hormone and a monoamine neurotransmitter, which is a compound that sends messages to parts of the body to influence important functions, and helps regulate everything from a person’s core temperature to their appetite and sleep cycles. Serotonin is a product of the essential amino acid tryptophan, which isn’t produced by the body, but ingested via foods such as chicken, cheese, fish, peanuts and turkey.

“Serotonin is a chemical that carries messages between nerve cells in the brain and throughout your body,” says Raj Dasgupta, M.D., a board-certified physician based in Los Angeles. “Serotonin levels that are too low or too high can cause physical and psychological health problems.”

While neurotransmitters such as serotonin are often linked to mental health, 90% of serotonin is actually found in the gut, adds Ben W. Holt, M.D., a psychiatrist at Intermountain Health’s McKay-Dee Hospital in Utah. “Serotonin helps regulate the movement of food through our digestive tract,” he explains. “Meanwhile, serotonin’s function in the brain can influence our mood, sleep, memory and ability to learn.”

Serotonin vs. Dopamine

“Serotonin and dopamine are both neurotransmitters that have important roles in the brain,” says Dr. Dasgupta. While serotonin is related to functions such as sleep regulation, mood, digestion and appetite, dopamine relates to the motivation, reward and pleasure sensors in the brain, Dr. Dasgupta explains.

Importantly, research suggests dopamine affects a person’s motivation and whether their brain believes that the reward will be worth the effort when completing a task. Lower levels of dopamine and a difference in the way dopamine is processed by the brain (such as dysregulation in the dopamine system) are characteristics often associated with attention-deficit/ hyperactivity disorder (ADHD), in which individuals may experience symptoms such as inattention and impulsivity.

While serotonin and dopamine serve different functions in the brain, both can contribute to mood disorders such as depression. Lower levels of serotonin may contribute to increased anxiety, sleep disturbances, increased cravings for comfort foods and overeating, and disturbances in cognitive performance and concentration, while lower levels of dopamine may lead to low mood and increased depression, fatigue and lack of energy, along with disturbances in sleep and increased appetite.

Serotonin plays a vital role in regulating various bodily functions, including the following noted below.

Digestion

Approximately 90% of the body’s serotonin is produced in the gut^[1]Bektaş A, Erdal H, Ulusoy M, and Uzbay I T. Does Serotonin in the intestines make you happy?. Turkish Journal of Gastroenterology. 2020;31(10):721–723. . Serotonin is produced by cells in the gastrointestinal tract, where it influences digestion by stimulating muscles, and can help protect against conditions such as irritable bowel syndrome. An increase in the levels of serotonin in the gut can cause digestion to speed up and also acts as a natural appetite suppressant.

Mood

Serotonin is an important factor in mood regulation and the reaction to external stimuli. While the link between serotonin and mood requires more research, a lack of the hormone and neurotransmitter is linked to irritability, lower energy and a depletion of pleasure.

Individuals with seasonal affective disorder (SAD) often have lower levels of serotonin in their brains, according to the National Institute of Mental Health. This is particularly apparent in those experiencing SAD during the winter months, when there is a reduced amount of daylight, which may lead to a reduction in vitamin D. Studies suggest that vitamin D influences the production of serotonin, making it even more key in those with depressive disorders^[2]Huiberts L M, Smolders K C H J. Effects of vitamin D on mood and sleep in the healthy population: Interpretations from the serotonergic pathway. Sleep Medicine Reviews. 2021;55:101379. .

Sleep

Serotonin is an important factor in sleep initiation, sleep quality and REM sleep (the phase of deep sleep associated with dreaming and cognitive restoration). Serotonin also plays a role in regulating the circadian rhythm. A small 2021 study of night shift workers linked poor sleep or sleep deprivation to lower levels of serotonin in the blood^[3]Shehata R S A, Nour Z A M, Khalifa E M, et al. Serotonin variations and sleep disorders among shift workers. A cross-sectional study. Toxicology and Industrial Health. 2021;37(10):603-609. . Serotonin may play a role in the production of melatonin (a hormone that regulates the sleep-wake cycle), as serotonin is a precursor to melatonin. Further research is needed to determine exactly how serotonin influences sleep.

Sex

Serotonin levels are linked to libido, with high levels leading to a low libido and lower levels leading to a high libido. This is due to the stimulation of certain receptors in the brain, according to Mental Health America, a non-profit organization dedicated to promoting mental health.

This association explains why sexual dysfunction is a common side effect of selective serotonin uptake inhibitors (SSRI), medications often prescribed for mental health conditions that alter the way serotonin is processed by the body.

Blood Clotting

Serotonin also impacts the cardiovascular system, as it works by constricting the blood vessels, which helps slow down bleeding and encourage clotting, notes Mental Health America.

Bone Density

Research suggests that higher levels of serotonin in the blood impact bone health, leading to normal or increased bone density. However, further research is required to examine the link between serotonin and bone health.

Some studies suggest people who take SSRIs may experience lower bone density, although further research is warranted^[4]Wadhwa R, Kumar M, Talegaonkar S, Vohora D. Serotonin reuptake inhibitors and bone health: A review of clinical studies and plausible mechanisms. Osteoporosis and Sarcopenia. 2017;3(2):75-81. .

Nausea

Research notes as the hormone originates in the gastrointestinal tract, serotonin is involved in the body’s nausea response, which leads to vomiting. Medications that block serotonin in the body are prescribed to treat nausea.

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“The normal range for serotonin in the blood is between 50 and 200 ng/mL,” explains Dr. Dasgupta. However, the levels of serotonin found in the blood don’t reflect the levels in a person’s brain, so the results won’t necessarily be helpful in most diagnoses, he says.

“Generally, serotonin is only tested if a carcinoid tumor is suspected, which would result in abnormally high levels of the serum,” explains Dr. Winter.

Having too little serotonin is related to an array of health conditions, such as depression and anxiety disorders, sleep problems, obsessive compulsive disorder and phobias, and symptoms such as digestive issues.

Low serotonin levels may be caused by a variety of factors. According to Laura Purdy, M.D., a board-certified family medicine physician based in Miami Beach, Florida and chief medical officer of OpenLoop, a digital health care company, Dr. Winter and Dr. Dasgupta, these factors may include:

• Genetic predisposition
• Nutritional deficiencies
• Medications
• Chronic stress
• Lack of sunlight
• Sleep deprivation
• Certain medical conditions
• Poor gut health
• Lack of exercise
• Aging
• Hormonal changes

Too much serotonin can result in serotonin syndrome, which is potentially fatal. This is usually caused by ingesting excess amounts of prescribed medication, interactions between medications or from using recreational drugs, which affect the production and effect of serotonin on the body. An excess of serotonin leads to toxicity, and may cause a number of debilitating symptoms, including:

• Fever
• Nausea
• Vomiting
• Diarrhea
• Increased heart rate
• High blood pressure
• Agitation and restlessness
• Confusion
• Hallucinations
• Seizures
• Tremors

“If you start to experience any of these symptoms then it’s best to seek medical care immediately,” explains Dr. Purdy.

There are a number of ways to increase serotonin if you’re experiencing symptoms that may indicate low levels. According to Dr. Purdy, these include:

• Medication: While selective serotonin reuptake inhibitors don’t increase the amount of serotonin in your body, they do alter the way the brain processes it. As a result, they’re often prescribed to treat conditions that may be affected by low serotonin levels.
• Vitamin D: Studies show that vitamin D has an impact on serotonin regulation in the brain. Spending time outdoors in sunlight can help the body synthesize more vitamin D. Incorporating more foods that naturally include vitamin D can also help boost levels of the vitamin. These foods include red meat, egg yolks, liver and oily fish, such as sardines and salmon. Some people may also need to take a vitamin D supplement to raise their levels.
• Balanced diet: Foods that boost the production of the essential amino acid tryptophan, such as eggs, cheese, tofu, salmon, turkey, nuts and seeds, may aid in increasing serotonin levels, explains Dr. Purdy.
• Exercise: A 2017 study in Brain Plasticity notes that acute exercise boosts serotonin, and that the more strenuous the exercise, the more serotonin is produced^[5]Basso J C, Suzuki W A. The Effects of Acute Exercise on Mood, Cognition, Neurophysiology, and Neurochemical Pathways: A Review. Brain Plasticity. 2017;2(2):127–152. . Mental Health America recommends exercising for at least 20 minutes each day to boost serotonin.
• Light therapy: Serotonin production is influenced by exposure to bright light, meaning it’s important to get a lot of natural light or use a light therapy lamp to boost the neurotransmitter. “Getting lots of sunlight is also a natural mood booster,” says Dr. Purdy.
• Stress management: Dr. Purdy notes that lowering your stress levels may have a beneficial effect on your serotonin levels. “Try to manage your stress levels, spend time with people you love and seek help from professionals to talk,” she explains.

If you notice your mental health worsening or are experiencing problems with sleep, Dr. Winter recommends discussing your serotonin levels with a health care professional.

“If you are experiencing moodiness, depression, insomnia or anxiety, it’s best to connect with your doctor,” says Dr. Purdy.