I brush his teeth every day, so it's not a teeth/gum problem. They're pretty
OK for a 17 years old cat. I doubt hyperthyroidism could have made him lose
weight so fast. Diabetes no. He doesn't drink enough. He's peeing regularly
and his pee doesn't smell strong. I don't believe it's his kidneys either.

When the weather goes down, he eats more. I would think maybe his heart is
giving up, which makes digestion more difficult and make him puke. Though some
pages on the net pretend that "120 – 240 beats" per minute is a normal rate,
180 at rest for my cat is certainly fast.

When the weather goes down for good, I believe he's going to be better. When I
feed him small portions of food many times a day, he eats, not much but enough
not to lose more weight, I suppose.

Unfortunately, at 17, I don't believe he still has a long life ahead of him. I
know some people here have cats that grew to be 50, but all the ones I have
known died around 17-18, even with good veterinarian care. Last year, a
veterinarian on TV seemed very proud because she had got her cat to 20 years
old... after 20 years of veterinarian care, being told "You can't eat this
kitty, it's not good for you."

So, my expectations are limited, but soon as we keep teasing each other,
things are not too bad, I suppose. As I write this, he's meowing to have
something to eat, so I'll have to go.

Thanks everybody for so much good advice about sending my cat to the vet to
get him to 50 years old. It's been very instructive. But, for now, I believe
it's going to be pretty much it.