I just registered a few days ago and am very happy to find this forum.
Welcome to the Cat Forum. We're glad you're here and are always glad to help answer questions where we can.
Spock, is 12 years old and has no symptom of hyperthyroid as far as I know and I observe.
I'm sorry to hear of Spock's diagnosis, but it's actually a very good thing that he has been diagnosed, because untreated hyperT can be deadly. Fortunately, hyperT can be successfully managed or even cured in most cats, depending on the treatment you choose. The most common symptoms of hyperT are ravenous appetite coupled with weight loss. These cats may also be more vocal and hyperactive than usual, and may drink water excessively. As you already know, cats with uncontrolled hyperT may also develop heart murmurs, but these can sometimes resolve once the hyperT is brought under control. Occasionally, a cat can have hyperT without any of these symptoms, or with entirely different symptoms.
The blood test showed that whatever value to determine hyperthyroid is 7.4 although the normal range is between 0.8 and 4.0.
This does indicate that he is hyperT, but it doesn't necessarily tell you anything about the severity of the disease or how long he may have had it. Let me offer you a little hyperT primer so that you'll understand more about it and your treatment options.
HyperT is a complicated disease because it puts the entire body into a state of hyperactivity. This includes the heart and kidneys (more about this later). It seems that many vets are not well versed in hyperT, its testing, or its treatment options. I am pleased to see that your vet has already recommended either daily medication or radioactive iodine treatment (I-131), as those are the preferred treatment options.
I-131 is the treatment of choice for hyperT cats with healthy kidneys because it is CURATIVE in almost all cats who undergo the procedure. It involves a single, subcutaneous injection followed by a period of quarantine at the administering hospital while the radioactive material dissipates to a safe level. This is a very costly treatment up front, though the cost is comparable to or less than the medication alternative over the long run, and again, it is the only CURE for hyperT.
HyperT can also be managed, but not cured, with medication – either methimazole or carbimazole. As you’ve already discovered, medication is not risk-free. Some cats experience unpleasant side effects that can range from mild to life-threatening, though many cats remain on medication for many years with no problems at all.
Although I have no direct experience with or knowledge of Resthyro, I doubt it is effective at either treating or managing hyperT. I recommend referring that question to the hyperT mailing list (linked below). I have gained most of my information and knowledge about hyperT from the mailing list, its members, their posts and message archives, and the research papers and other educational files stored in the "files" section of the list's Yahoo website. I strongly recommend you join that list. Even if you prefer not to post, you can learn a great deal by doing searches for specific information in their message archives and by reading the uploaded files. You may also, of course, ask whatever questions you may have of the list members by posting to the list. I'm sure there are members of the list who have researched and/or used Resthyro who can answer your questions about it. You can read about and join the list through this link:
feline-hyperT : A support list for people whose cats have been diagnosed as hyperthyroid (hypothyroid also welcome).
Let me back up a bit now and elaborate on hyperT and the meds used to control it. The Total T4 test does not test the actual function of the thyroid gland. It merely measures the amount of T4 thyroid hormone circulating through the bloodstream at the time of the test. It cannot tell the vet whether the T4 hormone level is the result of excess hormone accumulating in the bloodstream over time as a result of mild hyperactivity of the thyroid gland or whether it accumulated quickly as a result of more severe hyperactivity of the thyroid gland.
Some vets, however, seem to think that a very high T4 indicates the severity of the disease (which it does not - it ONLY indicates a very high level of circulating thyroid hormone in the blood)), and therefore prescribe too high a starting dose of methimazole. Methimazole itself does nothing to eliminate excess circulating hormone. Methimazole only helps to limit the amount of new hormone released by the thyroid. So all of that excess circulating hormone is either used by or eliminated from the body naturally and gradually after Methimazole therapy is started.
Here's the problem. If the starting dose of Methimazole is too high and shuts down the thyroid production too far, you won't know it until after the excess circulating hormone has been naturally utilized or released from the body, and that can take 1-3 weeks after starting the too-high dose. After that 1-3 week time frame is when a "crash" may happen ... if it's going to happen. If a crash happens, your cat will become hypOthyroid and may become extremely ill - nauseous, inappetant, weak. Usually this condition can be reversed by lowering the Methimazole dose, but occasionally the cat simply cannot recover and will fail.
A while ago, someone posted on this forum about their newly diagnosed hyperT cat. I warned him about the high starting dose of methimazole that his vet had put his cat on, and I begged him to start at a lower dose and work up to a higher dose, if necessary, to prevent his cat from "crashing". Unfortunately, he chose to ignore my warning and go with his vet's recommended dose. A few weeks later, his cat crashed so badly that he had him euthanized. It was a tragic and, in my belief, totally preventable loss of life. I don't want to see that happen to another cat.
Vets generally recommend a starting dose of 5-10 mg of Methimazole daily (generally split into two daily doses). This is the dosing recommendation they read in their standard veterinary pharmaceutical text. Unfortunately, many people, including myself, have found that even the lower end of this dosing recommendation is too high for our cats and has thrown them into hypothyroidism. Luckily, I was able to bring my cat back to euthyroid (normal thyroid levels) by lowering the dose. Others weren't so lucky with their cats.
On the hyperT mailing list, and in some veterinary research literature, you will find a much safer recommendation to start methimazole at a dose of 1.25 mg twice daily (1.25 mg once daily if the cat has known kidney disease), with retesting in 3 weeks. If euthyroid has not yet been achieved, a small dose increase of 1.25 mg daily is added and the cat is retested in another 3 weeks. This 3 week schedule of retest-med adjustment continues until euthyroid is reached.
This "start low and go slow" approach to dosing offers several critical advantages. First, it allows the body to slowly adjust to the decreased thyroid hormone. Second, it helps avoid severe crashes and hypothyroidism from occurring. Third, it protects the kidneys from the potentially devastating effects of a crash. There is NO disadvantage to starting on a low dose of Methimazole and working your way up, as necessary. It is the safest approach for your cat and one I hope you will discuss with your vet if your vet prescribes the standard starting dose of 5-10 mg daily.
Most cats can take methimazole with no or few side effects if dosed with the “start low and go slow” approach, though some may have a more difficult time with the med. Vomiting, inappetance, and facial itching are the most common side effects that some cats may experience. If these side effects occur, there are strategies such as always administering the med with a meal, using a transdermal form of methimazole or switching to carbimazole, which tends to have fewer digestive side effects.
Kidneys are a primary concern when starting thyroid med therapy because hyperT can mask renal failure. HyperT makes the kidneys work in overdrive, and blood tests cannot differentiate between healthy kidneys performing normally and sick kidneys being forced to work harder by the effects of uncontrolled hyperT. Both healthy kidneys and hyperactive ailing kidneys may have blood values within the normal range, so you can't know whether or not your cat's kidneys are truly healthy until AFTER you have normalized your cat's thyroid hormone levels. If, at that point, his bloodwork still shows healthy kidneys, then you can rest assured that they really are healthy.
This is where things get really complicated. IF your cat's kidneys are ailing, hyperT can actually help them function better to some degree. But that means you will have to balance the management of both conditions (hyperT and chronic renal failure) so that the hyperT can continue to support the kidneys without damaging other body systems.
Let's assume for the sake of this illustration that your cat DOES have masked CRF in addition to hyperT. Think of it as a teeter totter. You have CRF on one side and hyperT on the other. Since you didn't mention anything about your cat's kidney values being elevated, I'll assume that they're currently in "normal" range in his bloodwork. Since T4 is elevated, that puts the hyperT side of the teeter totter high in the air while the masked CRF side is solidly on the ground - certainly not the level balance we're striving for. Now let's put I-131 on the same side as the hyperT. This sends the hyperT side of the teeter falling to the ground as it CURES the hyperT; but without the support of the hyperT-induced hyperactivity, the previously masked CRF is now unmasked and goes sky high - still no balance here, just traded one set of illness symptoms for another. So how do we find that balance we're looking for between the two illnesses? Thyroid med, that's how. Forget the I-131 and use the med instead. The med now goes on the side with the hyperT, and by carefully adjusting the dose, we can balance that teeter so that CRF and hyperT both hopefully hold steady at a level that maintains the best possible degree of health for the cat.
If you decide to consider I-131 treatment for your cat, it is VERY important that you do a thyroid med trial first to get your cat's thyroid hormone level stabilized in normal range BEFORE you make a decision about I-131 treatment. That is the ONLY way you would be able to tell whether your cat's kidneys are healthy to determine if your cat would be a good candidate for I-131.
Try not to let yourself be too overwhelmed by this diagnosis. HyperT is a disease that can be cured or successfully managed in the vast majority of cats. You’re already taking a great first step in educating yourself about it. The more you know, the better a position you’ll be in to partner effectively with your vet and advocate for the best possible care for your cat. If I were in your place (and I have been … twice), I would start my cat on a low dose of methimazole or carbimazole immediately to start bringing his thyroid levels down gradually, then retest in 3 weeks and go from there.
Hang in there. This is one dragon you can slay.