Serotonin is primarily produced in two main locations in the body:

• About 90% of the body's serotonin is synthesized in the gastrointestinal tract by enterochromaffin cells lining the gut. This serotonin regulates intestinal movements and is released into the blood, where it is absorbed by platelets

• Approximately 10% of serotonin is produced in the brain, specifically by neurons in the brainstem's raphe nuclei. This central nervous system serotonin plays key roles in mood, appetite, sleep, memory, and cognition

Thus, the majority of serotonin is made in the gut, with a smaller but crucial portion produced in the brain. Both pools serve different physiological functions.