When I got my current two cats, Katie and Cinders, last June, Katie
was a bit of a food-hog, and rather plump as a result.  Over the last
three months or so, I had noticed that Katie was no longer so plump,
but her fur, and the fact that she has never liked to be picked up,
kept me from realizing just how much weight she had lost until a
couple of days ago, when I realized that she had been much more
lethargic than usual for several weeks (I can only plead a bit of
self-centeredness, due to some recent medical problems of my own, for
taking this long to notice her lethargy).  She had become downright
gaunt.  I took her to the vet today, and discovered, to my shock,
that she had gone from 12 pounds (her weight last August) to under 6
pounds.  The vet did blood tests and took chest and abdominal X-rays.
Katie's chest X-ray looked OK, but there was a shadow in the kidney
area, she was somewhat dehydrated (even though I keep fresh water out
at all times), and her bowels had an unusually large amount of stool
in them.  The vet gave her a laxative, and is keeping her overnight
in order to collect urine and stool samples.  The vet also wants to
see if the firmness in the kidney area goes away once the apparent
constipation is dealt with (apparently the shadow could be caused by
the unusually-full colon).  Some of the blood tests were done
in-house, but others had to be sent to an outside lab, so the vet
won't get the test results until midday tomorrow.  If Katie's abdomen
still feels firm at that point, an ultrasound and/or biopsy may be
necessary.  I will post more details tomorrow once I know more.