Hello and thanks for trusting me to help you and Joe today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
While it is true that most cats with hyperthyroidism have good to voracious appetites, I have run across a few that did not. More importantly, It is possible and likely that the increased thyroid level was an incidental finding on his weight loss work up may be playing little to no role in the weight loss. What I am saying is that Joe may have an elevated thyroid level, but it his weight loss may be due to another disease that hasn't been identified yet. This is often the case in cats with underlying GI disease as well as hyperthyroidism.
GI disease, such as inflammatory bowel disease (IBD) or GI cancer, can be present and there can be no other abnormalities noted on bloodwork, xray or even ultrasound, although these tests should be run to rule out other conditions that present with the same symptoms. There is a specific GI blood panel that can be requested and if abnormalities noted, can lend support to either pancreatic or intestinal disease. The only way however, to confirm the diagnosis is with a biopsy of the intestine. This can be done via endoscope, but is better performed with an exploratory surgery. The problem with this is that many cat owners are not willing to pursue the diagnosis by these means. It is for that reason that many cats are "suspected" to have IBD or GI cancer as the cause of their weight loss and anorexia once all of the other easy to find things have been ruled out.
I would definitely recommend having Joe re-evaluated by your veterinarian so that the above can be discussed.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.