What happens when the serotonin level is too high?
The neurotransmitter serotonin fulfills numerous important tasks in the peripheral and central nervous system. Among other things, it is responsible for the contraction of blood vessels and the intestines and regulates mood, appetite, sleep-wake rhythm and body temperature. To perform all these tasks, a certain amount of serotonin is needed.
The body produces serotonin itself, mostly (about 90%) in the intestine, but also in the brain (about 10%), since the neurotransmitter cannot cross the blood-brain barrier and therefore cannot pass from the intestine to the brain. Serotonin production is usually steady and always such that sufficient serotonin is available when needed. Excess serotonin or serotonin that is no longer needed is broken down by the body and then excreted in the urine via the kidneys.
This system is in good balance, but it becomes unbalanced if serotonin levels are suddenly too high. This can happen when appropriate medications, such as certain antidepressants, are administered that increase serotonin levels. Also, an increase in dose or the combination of different medications can cause the amount of serotonin to be much too high, resulting in serotonin syndrome.
Serotonin level too high - What happens?
Too much serotonin in the body has a toxic effect that can have fatal consequences. When the concentration of serotonin is extremely high in the central nervous system, excessive amounts bind to the receptors and a so-called serotonin syndrome occurs. The first signs of this occur within a few hours, with most sufferers taking less than 6 hours to show symptoms . These typically include hot flashes, palpitations and an increased urge to move. In addition, there is sweating, tremors and muscle cramps, and confusion. Some patients whose serotonin levels are too high also suffer from vomiting or diarrhea. Urgent action is needed because, if left untreated, overdose can lead to death.
How high should the serotonin level be?
To avoid overdose and its dangerous consequences, it is important to monitor serotonin levels. This can be done directly by serotonin determination from blood serum or indirectly by measuring hyroxyindoleacetic acid (HIES), which is excreted as a degradation product of serotonin in urine. Reference ranges, i.e. normal values, are considered to be 50 - 200 µg of serotonin per liter of blood  and up to 9 mg of HIES in 24-hour collected urine .
Conclusion about the serotonin level
When serotonin levels are too high, serotonin syndrome occurs, with symptoms including rapid heartbeat, sweating, tremors, muscle spasms, and mental confusion. Without treatment, serotonin syndrome can be fatal.