How old is your cat? My last, darling Skimble lived into his 20th year, but eventually his loss of weight and other symptoms caused me to visit the vet with him for a terminal visit - yes, it was his kidneys. It took two lethal injections to cause him to sleep peacefully, because his kidneys were simply not functioning. The vet explained to me that 1) this was normal for his age and 2) that to let him die of kidney failure would be excruciatingly painful for him.
I cuddled him as he went to sleep for the last time - and I've started again with tears now so I'm leaving this thread. You may not be at this stage, but please, please be aware and do not let your cat suffer. :)