Dry eye disease can be a result of something bigger happening in the body.
One thing I look for when I have a patient with dry eye and dry mouth is something called Sjogren's Syndrome. This is an autoimmune disease that impacts the glands that produce tears and saliva but it can also impact many other parts of the body.
One test I use when I'm suspicious of Sjogren's is a Schirmer's test, which I'm holding here. It measures the amount of reflex tearing a patient has which in a Sjogren's patient is significantly reduced.
Yet another way the eyes are a window to the rest of the body!