One word of warning, I would do a full thyroid panel. I do occasionally see a cat with a normal T4 that has hyperthyroidism. A full panel would rule it out fully. It's an easy non-invasive test to make sure it's ruled out. These signs scream hyperthyroid, so it's worth ruling out.
In the case that it's normal, I would start to worry about intestinal lymphoma. That is something that is diagnosed with intestinal biopsies. Surgical exploratory and biopsies would be the next step. That is to look for any masses. If there are no masses in the abdomen, then we take biopsies samples of the gut, liver, lymph nodes, ect. In those biopsies, we should have an answer. It's likely something microscopic, so biopsies are needed to make the diagnosis.
Treatment then depends on what is found. My guess is that it's either Lymphoma or IBD (inflammatory bowel disease).
Glycogen storage disease is seen rarely, but always in kittens shortly after birth or within the first year of life. Never this late.
I hope this helps, let me know if you have any other questions.