One of my cats has stopped using the litter box to urinate. I have two cats and three boxes; two are in one room and one is in another room. She uses one of the boxes in the first room to defecate, but she won't pee IN the box; instead, she pees right beside the box. I know she doesn't like sharing a box and my male cat uses both boxes, but he doesn't use the third box in the other room and, while she has used it once, she mostly ignores it. I scoop 3-4 times a day, I change the litter regularly, but it's getting frustrating and very expensive to have to line four litter mats with puppy training pads and change them all the time.
Her last set of labs showed that her hyperthyroidism is under good control, and her creatinine is normal, but her bun was slightly elevated at 51 (CK was 3400). Could the beginnings of kidney disease be causing this and, if so, is there anything I can do about it? Or could it be something else?