Now we do need to tread with care here for Lucy.
I am glad to hear that bloods were good post operatively, but we may want to consider a recheck to just make sure that low thyroid levels (as they can cause skin issues) and that there is nothing immunosuppressing her and allowing opportunistic agents to take advantage of her.
First, I would note that appetite reduction isn't actually unexpected after the removal of an overactive thyroid. The excess thyroid hormone often makes them excessively hungry and its removal will reduce that huge appetite. So, her current appetite may not be abnormal for her, it just depends on how much she is actually eating.
That said, if she is vomiting more often, that could suggest lurking nausea and that could impact the appetite. Since you noted it was mostly hair, do consider treating her daily with a hairball paste/gel +/- adding a bit of fiber (ie spoonful of canned pumppkin, 0.25tsp Benefibre) to her food. These will help push hairballs out the other way and may settle the vomiting. Of course, if it lingers, anti-nausea medication from her vet would be ideal for that.
Otherwise, even if that appetite is normal, she will lose weight if she has diarrhea. Therefore, you are right to have her on the Prokolin but we may need to increase that dose, worm her (ideally using Panacur since it treats against worms and some protozoal causes of diarrhea) and/or test a stool sample. That way we can pinpoint the cause of her continued loose stool, address that, and get her body using those lost nutrients to maintain her own weight.
As well, since she is losing weight and potentially eating less then she should, we do want to ensure we are getting as much nutrition into her as possible. To do so in a cat we cannot get to eat more, there are a few "cheats" we can use. Specifically, you can consider feeding or at least supplementing her with a calorie rich diets (ie Hills A/D, Royal Canin Recovery diet, even canned kitten food). You could also add in a liquid diet (ie Clinicare, Catsure) or there are paste supplements (ie Nutrical) that can also be used. And these will all get more in per bite to help counter that weight loss.
Finally, in regard to her skin and eyes, I would note that the eye discharge is just likely due to her immune system being bombarded by everything else. So, you can flush this with sterile saline (ie OTC first aid eye wash, plain contact lens solution), but addressing the other issues will help with this as well. Otherwise, her neck scratching is a more subtle issue. Since it has been months since the surgery, we'd assume this isn't related to that. Therefore, we could be looking at an allergic skin irritation (in reaction to something she is in contact with or a dietary allergy) but it can also be a sign that her mouth is bothering her. At her age, dental disease, oral ulcers (secondary to kidney disease) gingivitis, oral infection, and oral growths could all cause discomfort that could in turn reduce appetite and cause her to persistently scratch where she can reach. And the signs may be more subtle for her because the Metacam will be masking the problem a bit. Therefore, considering her age and all her signs, I'd be keen for her vet to check her teeth and mouth. That way we can head off any problems with that and if need be they can dispense stronger pain relief, antibiotics, and appetite stimulants as needed to help settle her multiple signs and help us get some weight back onto her.
Please take care,
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