One reason I stopped posting photos of my cat here - oh, I still took them - was because it was depressing me that she kept getting thinner.
Well, she got a lot thinner this year, and it turned out that she was extremely hyperthyroid, and I'm pretty sure the fact that I dedicated a month to getting as much food into her as possible actually kept her alive. But I got her to the vets, and treated her, and after two months of vet visits and medicine and being hyperaware of the cat, and finally losing her for a week to isolation after a radio-iodine injection, I can finally say that she has gained all the weight she lost in the last three years back - in three weeks.
Hyperthyroid cats have overactive thyroids, which means faster metabolisms, which means that they need to eat a lot more than usual or starve. It appears in old cats and comes with a charming array of other problems.
If you think your cat might have hyperthyroidism, you may benefit from reading the article I wrote up about it and my experiences: Feline Hyperthyroidism: Why My Cat Started Starving to Death (even if you don't read mine, I strongly recommend doing some reading around before talking to the vet).