I highly recommend taking your cat to a feline specialist or internal medicine specialist. There are four common causes of diarrhea in cats of this age, but treatments are very different, so diagnosis is essential. I agree with Dr. Shaw about the diagnostics, GI panel at TAMU, T4 (and possibly Free T4 by ED), and biopsies.
The most common causes for intractable diarrhea in a cat this age are:
- Small intestinal inflammatory disease (easily treated with vitamin B12 if there is a deficiency, diet change and antiinflammatories).
- Small cell lymphoma (easily treated with prednisolone and chlorambucil).
- Hyperthyroidism (easily treated with Methimazole, radioiodine, or surgery).
- Intestinal adenocarcinoma (Usually treated with surgical excision).