Does anyone want to armchair diagnose my cat?

[IToldYouIWasFreaky]

My 13 year old cat is currently at the vets.

For the past few days, he's been off his food which is very unusual for him as he's a right guzzler! He also peed in the kitchen earlier this week...this is not so unusual for him as he does have previous for doing small, scent-marking type wees in the house. He was checked out at the vet for this last year and they found nothing wrong.

He's generally not been himself, quite listless, and his fur looks a bit manky, like he's not been grooming himself properly.

So, off he went to the vets this morning. The vet suspected thyroid issues but did a full range of blood tests and they've come back clear. He's currently on a drip for dehydration and he'll stay in over night. He may have an ultrasound tomorrow if there's no improvement. DP has been dealing with the vet so I'm getting all of this second hand and he didn't ask what the ultrasound might be looking for, or find...

We'll obviously find out from the vet tomorrow what the issue is but we're worrying in the meantime - does anyone have any idea what it might be?

[Want2beme]

It's so difficult with cats, because they hide everything so well. Does he have anything wrong with his mouth?

I hope he's on the mend