Could my cat's hyperthyroidism symptoms actually be another illness?

If thyroid levels are in normal range then the most common differential diagnosis is Lymphoma which is a type of cancer, unfortunately it is more difficult to diagnose, an experienced ultrasound specialist vet will be able to confirm or rule it out.

I'm sorry you lost your other cat and that Bird is now showing symptoms of another disease. The most common diseases in older cats are renal failure, hyperthyrodism and diabetes. It is really not a wise decision to give Bird any of the other cats medications. Just because you think she looks like she has symptoms of hyperthyroidism doesn't mean she actually has it. As a vet, I have been fooled many times. You could certainly kill her by doing this or make things worse. Even if she is hyperthyroid she may not need the same dose that the other cat was on. If she is that stressed getting to the vet I would ask your vet if they would be willing to make a house call to draw blood as that is the way to diagnose this issue. The other option is to find a mobile vet in your area that could come to your house. I hope this helps. If you have any additional questions please feel free to ask!

Hi there. Hyperthyroidism can cause symptoms like increased vocalization and irritability, weight loss, increased appetite, vomiting or diarrhea, and/or increased drinking and urination. It also contributes to high blood pressure, and is hard on the heart and kidneys long-term. If Midnite's thyroid levels are high, treatment would generally be recommended to address this.

These are indeed an unusual set of symptoms, thyroid disease and diabetes would be the first suspects for a cat with increasing appetite and weight loss. A specialist ultrasound scan of the abdomen may be worth while to investigate difficulties with food digestion and absorption or even tumours using up the calories

Poor Willow! I'm sorry she hasn't been feeling well. Generally speaking, yes - it's perfectly fine for a cat to be on medication for hyperthyroidism as well as insulin for diabetes. These two illnesses are both common in older cats, so many cats are on treatment for both problems. But, it isn't normal for Willow to be acting tired and miserable since starting her thyroid medication - this indicates that something needs to be changed. Some cats get an upset stomach and don't feel well on the oral tablets, but may do better with a transdermal gel or other formulation. Other cats simply don't tolerate methimazole (the medication) well no matter what, so other treatment options may need to be discussed. In rare cases, we can see potentially serious side effects with this medication such as bone marrow suppression, which could also cause lethargy and weakness - so it would be a good idea to have blood work rechecked with your vet as well to make sure that this isn't the problem. I would recommend calling your vet today to discuss the issue and see what they would suggest as the next step.

That is definitely a top thought with what you are describing. A simple blood test can confirm this. Other options do include gastroenteritis or an inflammatory bowel disease also so an ultrasound would be another useful test. Each have different treatments and they can be managed

There are many potential causes of diarrhea in a cat, and to treat the diarrhea you would need to identify the cause. Hyperthyroidism could certainly cause diarrhea so if you are waiting on lab work, or waiting for thyroid medication to take effect you may want to give it a little more time. It sounds like your veterinarian has potentially recommended some other treatments for diarrhea like a diet change to rule out a food sensitivity. Inflammatory bowel disease, infectious diseases or an intestinal mass could also cause diarrhea, so you may want to talk to your veterinarian about what the next diagnostic step might be - they might recommend an abdominal ultrasound or advanced fecal testing. If you have not tried a probiotic or antibiotics for the diarrhea you may want to ask your vet if they would help while you're waiting on diagnostic test results.