Yes, Siamese tend to be long and slender to begin with, and so when they lose weight, it is alarming.
Our Siamese "Pearl" was always a finicky eater and on the slim side. When she became anorexic from Cancer, we tried all kinds of cat food to find one she would eat. We literally went thru 50 different kinds of can cat food to get her to eat. The high liquid content in can cat food is good for her kidneys--and we have to watch out for high protein content too as our cat has problems w/urinary tract infections due to the chemo and old age..

I hope your cat makes a full recovery........

Pearl's Mom