Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I understand that you are concerned because Sissy Boy is getting very thin even with a normal appetite.
When we see weight loss with an apparently normal or increased appetite we need to worry about disease processes that either don't allow him to digest and/or absorb the food he is taking (such as diabetes, liver disease, primary intestinal or pancreatic disease including cancers) or those that lead to a higher metabolic rate such as hyperthyroidism or cancer.
In an older cat the more common diseases we see include hyperthyroidism (a tumor of the thyroid gland that overproduces thyroid hormones) or if he were drinking more and urinating a tremendous amount, diabetes. Sometimes early on with diabetes we only see weight loss.
Both diabetes and hyperthyroidism are treatable or at least manageable.
Hyperthyroidism is controlled with daily oral medication (Tapazole, also called methimazole) or a very special diet called y/d both of which must be done for the rest of his life or treatment with radioactive iodine to selectively kill tumor cells which is usually a one time therapy. Your second veterinarian is correct in that we don't see hypothyroidism (low thyroid function) in cats, and anyway that would cause weight gain not loss, so I suspect that there was a misunderstanding with your original veterinarian. I suspect that your original veterinarian was treating him for hyperthyroid disease with Tapazole. It's possible that he may have another problem or that his dose of Tapazole wasn't high enough.
Hyperthyroidism is not painful, but elevated thyroid hormones can make them feel edgy or nervous and thus cranky, increase the heart rate and interfere with sleep, like drinking way too much coffee or energy drinks. Long term if left untreated it can lead to heart disease, hypertension, possible eye damage as well as kidney and liver damage. But if caught and treated many of these things can be reversed. His change in eye color can be due to hypertension (high blood pressure) and bleeding in his eye or increased vessel prominence due to hypertension.
Diabetes is controlled with diet and insulin therapy. It too can lead to hypertension and kidney damage, and thus bleeding in the eye.
If his thyroid levels truly were low, and he was losing weight then I would be concerned that he is suffering from euthyroid syndrome. This happens when an animal has a serious life threatening, chronic illness that leads to thyroid gland suppression, and thus low hormone levels. We need to look for the primary disease, not supplement thyroid hormones.
This tends to happen with organ failure, cancer or an over-riding systemic infection. I would be worried about particular types of cancer such as lymphoma, or multiple myeloma. It may also be related to an unusual variant of a viral infection called the dry form of feline infectious peritonitis. All of these can affect the eyes, and cause changes in eye color. That diagnosis may be more difficult to achieve in an inexpensive manner. These disease processes may require more specialized testing.
It sounds like he has had some blood testing done to achieve a diagnosis, but has his blood work been rechecked after starting treatment? If not that should be done to see what sort of effect treatment has had on him.
Ideally he should have a complete blood count and biochemistry profile with T-4 checked as well as a urinalysis. These tests often give us the information that we need.
If those aren't diagnostic then it is time to move on to more specialized testing. I recommend checking vitamin B levels to look for signs of intestinal disease, a test for pancreatic insufficiency called a TLI, and an abdominal ultrasound to look for signs of changes in the walls of the intestines as well as evaluating his organs and abdominal lymph nodes. He may need an endoscopy to collect biopsies of his intestines if they look abnormal.
Dental disease can be uncomfortable, but dental disease alone rarely leads to weight loss. While he may have had rotten teeth and infected gums that alone isn't likely to be the whole problem.
In the meantime you might try feeding him a higher calorie prescription food called Hills a/d or Iams Maximum Calorie from your veterinarian and make sure he has access to plenty of fresh, clean water.
Please let me know if you have any further questions.