Why is my cat with diabetes and hyperthyroidism so tired?
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.
Updated on September 23, 2025