Hello cat-lovers (and vets?!). I noticed a few days ago that my 8 year old cat has got his third eyelid showing in the inside corner of both eyes. I googled of course, and got a lot of different possible causes. He seems OK in himself, eating normally etc, one of the possible causes is apparently dehydration, so I've been trying to encourage him to drink more - as the only odd behaviour we noticed was a bit of dry poo.
What should I do? Take him to the vet (would need to pay excess on insurance), or wait and see if it improves? Do we need to be concerned?