What Serotonin Is and How It Regulates Body Functions

The neurotransmitter serotonin (sometimes called 5-HT because of its chemical name, 5-hydroxytryptamine) is a substance that occurs naturally in your body. As a neurotransmitter, serotonin carries signals along and between nerve cells (neurons). It’s found mainly in your intestines but also in your central nervous system (CNS), which includes your brain, and your blood platelets.

Serotonin appears to affect and/or regulate a number of body functions, including:

Digestion: Serotonin plays a role in your bowel function as well as in reducing your appetite as you eat. In addition, your intestines produce more serotonin if you eat something that’s irritating or toxic to your digestive system. The extra serotonin helps move the affected food along so it’s expelled from your body more quickly.

Blood Clotting: The platelet cells in your blood release serotonin when you have any kind of tissue damage, such as a cut. This results in vasoconstriction—a narrowing of the tiny arteries, or arterioles, in your circulatory system—which slows your blood flow as part of the blood-clotting process.

Bone Density: Studies have shown that bone density and serotonin are linked—specifically, high circulating levels of serotonin the gut may be associated with osteoporosis. In fact, research suggests that antidepressants, specifically selective serotonin reuptake inhibitors (SSRIs) are associated with decreased bone mineral density and increased fracture risk. This isn't a reason to stop taking your SSRI, but rather, to have a conversation with your physician, especially if you have other risk factors, such as existing osteoporosis, a family history, or you smoke.

Sexual Function: The increase in sexual desire that can accompany alcohol intoxication is believed to be due to low serotonin levels. On the other hand, decreased sexual desire can occur in people taking medications that produce higher-than-normal serotonin levels.

Mood: You could think of its effects in your brain as serotonin’s “starring role” in your body. Widely known for playing a major part in regulating moods, serotonin has been called the body's natural "feel-good" chemical, because it's involved in your sense of well-being.

However, that’s only true when your serotonin level is within the normal range. What happens when it’s low? Perhaps the best-known condition believed to be associated with low serotonin levels is depression.

The cause of depression is still not completely understood. However, since serotonin is a key factor in mood balance, it’s believed that low serotonin levels can lead to depression.

Not surprisingly, increasing serotonin levels with medication has become a major part of depression treatment.

Depression is associated with low levels of serotonin. Increasing the amount of serotonin in the brain seems to help brain cells communicate, which has the effect of reducing depression and improving mood.

One group of serotonin-based medications called selective serotonin reuptake inhibitors, or SSRIs is used to reduce the symptoms of moderate to severe depression by increasing the level of serotonin in the brain. SSRIs are the most commonly prescribed antidepressants worldwide.

The second group of serotonin-based medications for treating depression, called serotonin-norepinephrine reuptake inhibitors (SNRIs), are sometimes referred to as “dual-acting antidepressants.” That’s because they increase the levels of two neurotransmitters, serotonin, and norepinephrine in the brain.

Two older types of antidepressants, tricyclics and monoamine oxidase inhibitors (MAOIs), also contain serotonin. However, they are prescribed less often today because their side effects are more problematic than those of SSRIs and SNRIs.