Studies have shown high prevalence of social anxiety in Parkinsons disease and schizophrenia. In a recent study, social phobia was diagnosed in 50% of Parkinsons disease patients. Other researchers have found social phobia symptoms in patients treated with dopamine antagonists like haloperidol, emphasizing the role of dopamine neurotransmission in social anxiety disorder.Some evidence points to the possibility that social anxiety disorder involves reduced serotonin receptor binding. A recent study reports increased serotonin transporter binding in psychotropic medication-naive patients with generalized social anxiety disorder. Although there is little evidence of abnormality in serotonin neurotransmission, the limited efficacy of medications which affect serotonin levels may indicate the role of this pathway. Paroxetine, sertraline and fluvoxamine are three SSRIs that have been approved by the FDA to treat social anxiety disorder. Some researchers believe that SSRIs decrease the activity of the amygdala. There is also increasing focus on other candidate transmitters, e.g. norepinephrine and glutamate, which may be over-active in social anxiety disorder, and the inhibitory transmitter GABA, which may be under-active in the thalamus. Brain areas
The amygdala is part of the limbic system which is related to fear cognition and emotional learning. Individuals with social anxiety disorder have been found to have a hypersensitive amygdala; for example in relation to social threat cues (e.g. perceived negative evaluation by another person), angry or hostile faces, and while waiting to give a speech.
Recent research has also indicated that another area of the brain, the anterior cingulate cortex, which was already known to be involved in the experience of physical pain, also appears to be involved in the experience of social pain, for example perceiving group exclusion. Recent research also highlighted the potent role of the prefrontal cortex, especially its dorsolateral part, in the maintenance of cognitive biases involved in SAD. A 2007 meta-analysis also found that individuals with social anxiety had  hyperactivation in the amygdala and insula areas which are frequently associated with fear and negative emotional processing. Diagnosis
ICD-10 defines social phobia as fear of scrutiny by other people leading to avoidance of social situations. The anxiety symptoms may present as a complaint of blushing, hand tremor, nausea, or urgency of micturition. Symptoms may progress to panic attacks.Standardized rating scales such as the Social Phobia Inventory, the SPAI-B, Liebowitz Social Anxiety Scale, and the Social Interaction Anxiety Scale can be used to screen for social anxiety disorder and measure the severity of anxiety. DSM-V Diagnosis
DSM-5 defines Social Anxiety Disorder as a marked, or intense, fear or anxiety of social situations in which the individual may be scrutinized by others.DSM-5 Diagnostic Criteria with Diagnostic Features:

Marked fear or anxiety about one or more social situations in which the individual is exposed to possible scrutiny by others. Examples include social interactions (e.g., having a conversation, meeting unfamiliar people), being observed (e.g., eating or drinking), and performing in front of others (e.g., giving a speech).