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I would check for both. At this age, you don't want to overlook anything.
Kidney disease can cause blindness, among other problems, so you want to nip this in the bud as soon as you can.
Increased thirst can also be a thyroid condition, diabetis, kidney or liver disease or other problem.
Let me know if you have more specific questions. I fear that my answer is too broad.
You MIGHT see this with the new food choice but most commercial brands are pretty comparable where sodium is concerned.
I just lost my 17 year old cat last week to kidney disease. It is not a gentle way to go. I suggest that you do all that you can.
I am glad that I was able to help. And thanks for the bonus!
I hope you come back with all your pet questions.