Why is my old cat not eating, hiding, and showing signs of kidney disease?

With the symptoms you are describing I tend to test for advanced kidney disease or hyperthyroidism. I would encourage him to eat as best you can and get him to a veterinarian. With blood work and a urinalysis they should be able to give you a diagnosis and help direct you as to what is next

Unfortunately, this could be a number of things. The first thing I tend to think of in older cats in which renal disease has already been diagnosed is the development of a urinary tract infection, and especially if urinary signs (urinating outside the box, increased frequency of urination, increased drinking, etc) that should be considered. Kidney disease predisposes them to the development of UTI's, and they can quickly become serious in these patients as often times it's not a LOWER UTI (i.e. a bladder infection) but an UPPER UTI (i.e., involving the kidneys). At this point she's only missed one meal, so I wouldn't panic yet - especially because she did eat when you hand fed her. However, do be diligent and consider whether she needs another checkup (to include a urinalysis and potentially x-rays or ultrasound) if this keeps up more than 24 hours, since it's such unusual behavior for her.

It certainly sounds as though Grey is not feeling well! Inappetance, hiding and vocalization can indicate a number of underlying conditions, but hyperthyroidism is at the top of the list. I recommend taking Grey to her vet for an exam and lab testing (CBC, chemistry, T4/thyroid hormone, urinalysis) to get to the bottom of things. Cats can develop hepatic lipidosis (fatty liver disease) when the stop eating, so her condition is fairly urgent.

It is common for cats to hide when they don't feel well. She needs to be taken to a vet to have the lethargy and bloody urine addressed. They will do blood work to check her kidneys function and a urine sample to check for an infection. Weight loss and dehydration can occur quickly in older cats. Your vet can better guide you in her quality of life based on the diagnostics.

Your cat should be examined by a veterinarian for a work up and treatment. There are a lot of diseases that could cause those signs including diabetes, kidney disease, liver disease, gastrointestinal issues, thyroid disease, and cardiac disease. Your vet may need to run lab-work and take X-rays to diagnose.

This is very concerning and may not related to progressing kidney, thyroid disease or diabetes. An urgent vet check and urine and blood tests is recommended. Do not delay investigation as early treatment is required

It sounds like he should be examined by a vet to rule out a medical problem. He could have an infection, parasite, metabolic, and/or endocrine disease and your vet can run blood work to help diagnose and the determine if treatment is needed. Hope he feels better soon!