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She is 84 years old and is having excessive salivation
and she spends most of the time in spitting or clearing the saliva. In a lot of persons of elderly age such type of symptoms are seen and in most cases the investigations come normal and no significant cause is seen. With advancing age excessive salivation may be a feature of Parkinson's disease. Poor expression, slow movements, rigidity etc are seen. Alzheimer's disease may be other possibility. Due to the salivation she is not getting enough rest and this is causing aggravation of anxiety
and depressive symptoms. I would advise you to consult a Neuropsychiatrist for expert help. Thyroid
may be a cause but other features of thyroid abnormality are also seen. Best thing is to take opinion of a Psychiatrist.