Patients with social phobia produced too much serotonin in a part of the brain's fear centre, the amygdala, which makes the patient more anxious.

They found that patients with social phobia produced too much serotonin in a part of the brain’s fear centre, the amygdala. The more serotonin produced, the more anxious the patients were in social situations. A nerve cell, which sends signals using serotonin, first releases serotonin into the space between the nerve cells.
The nerve signal arises when serotonin attaches itself to the receptor cell. The serotonin is then released from the receptor and pumped back to the original cell. Not only did individuals with social phobia make more serotonin than people without such a disorder, they also pump back more serotonin.
We were able to show this in another group of patients using a different tracer which itself measures the pump mechanism, said researcher Andreas Frick. Frick added that they believe that this is an attempt to compensate for the excess serotonin active in transmitting signals.
Serotonin can increase anxiety and not decrease it as was previously often assumed, noted Frick. The study is published in the scientific journal JAMA Psychiatry.
Source-ANI