For whatever reason, you're question was cut off short! I hope your cat is doing ok. Im going to answer your question with what I know is going on so far - borderline hyperthyroid and fever.

So, if any cat walks into the clinic (with it’s person of course), and is hot hot hot, I'm squashing the fever down pretty quick. 105 or 40 degrees Celsius (Im Canadian) is hot enough. So, even before I know what is causing the fever, I’m dropping the fever. Why? Most cat’s with fever’s don’t eat at all, or enough. Once this happens, we have about 3, 4 or 5 days until the liver begins to get upset with the whole situation. The body fat rushes the liver and induces a hepatitis. And then the problems have doubled and what may have quickly solvable is much more complicated.

So the first thing I'm going to give after a physical exam is a judicious amount of room temperature SQ fluids. We often warm our SQ fluids for comfort, but in this case we need them cooler. It is simply a combination of hydration and the fact that they have a mini waterbed of fluids that might convect a small amount of heat away from them.  For some cat’s, this it all it takes and then I am planning the rest of their care or diagnostics.

The other reason I do this is that if the fluids don’t work within a few hours, I have now hydrated them, so any medications I use becomes safer for their kidneys. I will roll as many safety factors into a treatment plan that I can, and getting in this hydration largely covers the kitty here. So if kitty cat otherwise is stable (i.e. not in shock, normal cardiovascular system and perfusion of blood to organs, hydrated), I may use an NSAID if indicated (a non steriodal anti-inflammitory as described in Zack Vs. Metacam:

What does “if indicated” mean? If they have not been eating for several days and they are hot! I usually have bloodwork done (unless some one has zero cash and this cat needs relief pronto) prior to introducing an NSAID into their body.  Because sometimes they need something else entirely to deal with the fever that isn't compatible with an NSAID, and then I'm wasting time getting to the right answer or therapy. But food is the big decider here. If they are eating well, we often don’t need an NSAID and vice versa.

If I have to use an NSAID I don’t use Metacam for this function, unless I’m sure there is a pain situation which is primarily causing the hyperthermia (which is an elevated temperature, but not necessarily a true fever). While Metacam is labelled as an NSAID, it is not labelled as an anti-pyretic, and in my experience it’s not the greatest at reducing fevers. It can work, but I want those fever kitties to go from anorexic, mopey and depressed to becoming purr machines and headbutting the kennel door quick quick quick. Ketoprophen is labelled as an anti-pyretic, and I will use that in hydrated cats for the specific purpose of fever reduction. Onsior is another NSAID that I have not used extensively yet, so I don’t know where it falls in it’s anti-fever activity over hundreds of cases or so. In a hydrated cat, I’m typically not going to use this type of medication more than a few days…because next we are digging further into the underlying cause, and hopefully resolving that instead of artificially dampening down the fever.

Worried about using an NSAID medication in your cat? Remember the liver conundrum as stated above if the fever is preventing them from eating. That is always my priority. My side effect rate with NSAID'S is very very low. The possibility of hepatitis developing and complications in treatment are much much higher. I know which one I choose!  

So…fever cat. Hopefully I have stabilized you by now, and just as importantly made you feel better so you will eat. Now what gave you the fever?

The list is gigantic. There is a small quantity of common things which are easy to find, but then there are hundreds of potential infectious and noninfectious causes of fever. Which is why we invented the term “fever of unknown origin” for cats. Which means the list of potential causes is so big, that I don’t have diagnostics available to me to find the reason.
But I know I have to help you find the reason and protect the organs at the same time. So bloodwork (including FeLV, FIV and FPli testing for pancreatitis), urinalysis and X-ray is often done to begin. Combined with the physical exam, I should be able to start narrowing down the list pretty quickly. And then get to a therapy that can help.

This isn't the only the approach to fever. There is both professional and common advice that unless the fever is life threatening (getting past 42 degrees or so), let it ride out while you are trying to do your diagnostics first. The argument suggests that anything you do to dampen down the fever is just artificially treating the patient, somehow making it more difficult to diagnose them.

I understand this approach logically. But you know what? I didn't become a vet to watch cat’s and dog’s suffer in front of me when I have viable solutions less than 8 feet away from them on a shelf! The welfare of our pets should be just as important when figuring out the right approach. And I’ve never personally witnessed that the act itself of reducing a fever (unless one throw’s antibiotics at everything at first sight) complicates the diagnostics. You know what complicates things real fast? The development of liver disease because they won’t eat enough because they feel awful.

Do they have a fever that is lasting than more than a few days? These guys are often with me in the clinic on IV fluids with whatever indicated medications they need, or they are getting daily room temperature SQ fluids at home while we are sorting out the underlying cause. Lot’s of nursing care and making sure they eat enough. They won’t eat at all and your past a week? You maybe need to consider the tube ( or another solution like it. And digging deeper into the diagnostics won’t hurt.

P.S. - If they recently did bloodwork and found he is borderline hyperthyroid at the same time as having a fever, there is a chance that he is truly hyperthyroid. When you are ill, your thyroid hormone dips down. If your body is dipping it down, and you test borderline high, chances are it’s really high aka hyperthyroid. It’s can be super tricky to determine, especially if you have some chronic illness that keeps on dipping it down! Will hyperthyroidism cause a fever on it’s own? Not typically, but given that excess thyroid can be toxic to many things in the body, I guess never say never.  

Good luck,

Dr. Kris