Hi! Is your cat indoor/outdoor? If she also goes outside, she could've eaten something from outside or drank water from a puddle and gotten a parasite.
And, one other thing comes to mind. I believe that an overactive thyroid can cause vomiting and loose stools. Is your cat taking thyroid meds to control her thyroid issues?
Has she had access to anything like grass, or fine thread, or string? Once one of my cats had an issue like this, with a lot of vomiting over time. She turned out to have some grass up under her soft palate, which was stuck up there and she couldn't vomit it out. A thread would have the same effect. The vet had to knock her out and physically look for it, but he did find it. Certainly before you would put her to sleep, you should ask the vet to do this.
you said your kitty had a thyroid problem, is she on Iopanoic Acid? you mentioned an iodine product(?).....if the dose is too high it can cause vomiting...but already one of the symptoms of hyperT is vomiting, so it maybe a problem to control....sorry I can't be of more help, if I find more info I'll post it later.
http://www.marvistavet.com/html/thyroid_treatment__oral_medica.html
Thank you so much for the reply and articles! Mya was always on dry food, but when the vomiting became frequent we switched her to wet. She threw that up as well, and we switched her to a special iodine vomit form the vet, for her thyroid, but after a few fine days she is now throwing up again.
It's definitely vomiting, she often has bile when she throws up- sometimes there is still food/partially digested food in the vomit.
I might give her that cat grass, though, do you think it will help with vomiting?
heres a further link to Dr. Beckers discussion, on why cats vomit....good reading in both articles..
http://healthypets.mercola.com/sites/healthypets/archive/2010/11/09/vomitting-pet-cat-health.aspx
is she on a dry food diet?? sometimes on kibble they tend to gobble the food down to quickly and its not digested properly and causes them to vomit it up...if so try a canned diet.
heres a site you can read over with some additional tips from Dr. Becker. let us know how it goes, no need to put kitty down she's not showing any signs of it causing her any distress as of yet, she will continue to loose weight if the issue continues and become ill at some point.
http://healthypets.mercola.com/sites/healthypets/archive/2012/12/17/vomiting-vs-regurgitating.aspx?e_cid=20121217_PetsNL_art_1