No, I'm not a vet. I'm a guy who has lived most of his life with cats and who has watched too many die sooner than they should simply because there was something that I didn't know. The information and opinions here are not intended to replace working with a vet, and if you have a hyperthyroid cat (or think you might), I strongly suggest that you join one of the groups listed below. I will sometimes disagree with your vet, but I cannot guarantee that anything here will work for you. What I ask is that you do some real thinking. I'll give references and reasons, but those aren't enough. Check what I say. The vet has a degree, but that isn't enough, either. Check what the vet says. Cats are quirky, something you've probably noticed, and that can make diagnosing and treating them a combination of science, empathy, experimentation, and luck.
The two groups listed below are almost certainly the largest accumulations of experience on the subject of feline hyperthyroidism and related problems. Some of the members have been working in the groups for many years, gathering information from many sources in addition to the results seen in the groups themselves. In addition, the people in the groups are largely those who will understand what you're going through, not the type to say, "just a cat". Even if things are going well on your hyperT adventure, I really suggest joining one of the groups. For what it's worth, I spend more time on the Facebook group but try to stay involved with both.Facebook Hyperthyroid Cats Group Yahoo Feline Hyperthyroid Group
Feline hyperthyroidism evidently didn't even exist until the 1970s, maybe the late 60s. In the late 70s, cats were suffering from a wasting disease of unknown cause, although it was often put down as simply “growing old”. Some of the vets with such patients were asking around (yes, even before the internet), and word of a case eventually reached Dr. Mark Peterson, a young vet specializing in endocrinology who thought the symptoms suggested thyroid issues. He asked a friend at a hospital to do a scan, which spotted the tumor, and the mysterious disease finally had a face and name.....and the beginnings of a treatment.
It appears fairly conclusive that the cause of feline hyperthyroidism is the PBDEs used as flame retardants in everything from fabrics to building materials. Dust from those chemicals is now so widespread that it's even in the soil. There appear to be some other things that can disrupt the thyroid enough to cause a slightly high T4, but this should not be confused with feline hyperthyroidism. Feline hyperthyroidism is caused by an almost always benign tumor on the thyroid.
For more about the discovery of feline hyperthyroidism, read this entry on Dr. Mark Peterson's "Insights into Veterinary Endocrinology" vet/tech blog or "The Mystery of the Wasting House-Cats in the New York Times magazine, May 16, 2017..
With the discovery of a tumor on the thyroid, the initial treatment was surgery. Removal of the tumor resulted in a return to health, confirming that the thyroid tumor was the cause of the problem, hyperthyroidism. After that confirmation, dosing with methimazole began as a less expensive option. The initial philosophy was to bring the T4 down as quickly as possible, and then reduce the dose as needed. With that in mind, the earliest methimazole protocol called for 5mg three times a day. It wasn't a good idea, and that protocol was short-lived. (Be aware that a few vets will still use this protocol. Don't.)
Although the starting protocol was reduced to 5mg twice a day, the philosophy of dropping the T4 fast and adjusting afterward remained, and that stayed the protocol for over 20 years. The problem with the “fast” protocols is that T4 and kidney function are closely related. A high T4 increases the metabolism, which increases flow through the kidneys. This can cause the creatinine to read lower on blood tests, giving an incorrect picture of kidney health.
When the T4 drops, the metabolism slows down. If the dose of methimazole is too high, the T4 will drop below normal, into the hypOthyroid range. In this case, the kidneys may not process toxins fast enough, causing a back up that can lead to acute kidney disease. Dropping the T4 too fast may have the same result even if the T4 is still above normal. Systems simply don't like rapid changes and can't adapt quickly enough.
Sometime after 2000, the dosing philosophy began to change to lowering the T4 slowly and increasing the dose of methimazole gradually. After all, the disease comes on slowly, and in most cases cats are not in immediate danger. (There are some cases where a particularly high T4 can present a more immediate danger to the heart or blood pressure, and dropping the T4 somewhat faster at first may be the better choice.) This change in philosophy initially resulted in changing the starting dose to 2.5mg twice a day, and that is still a common protocol, if problematic.
By 2007, some official sources were saying that 1.25mg twice a day was a safer starting protocol, and the experience in both the Yahoo group and Facebook groups has strongly reinforced that approach. At different times, the 1.25mg protocol has been the number one or number two listed in Plumb's Veterinary Drug Handbook. However, when Dechra sought approval for Felimazole (methimazole for cats), they only tested doses at 2.5mg levels, the pill size they wanted to introduce. They ignored the already documented 1.25mg protocol completely. The Dechra studies are flawed in other ways, but the medicine was approved and marketed at that dose, and that may be why the 2.5mg protocol is now most common even though the studies showed that it's too much for the majority of cats.
Surgery remains a fairly popular option outside of the US but is rarely done in the US now. The I-131 has been increasingly refined and can now even be an option for cats with chronic kidney disease if special protocols are followed. However, it remains common to do a trial period with methimazole to be certain of kidney condition before doing the 131.
Several years ago, Hills introduced their Y/D food, but it really can't be considered much of an advance, only a last resort. The food only "works" if it's the entire diet, no other food or treats, and it's questionable nutritionally (low meat protein even in the canned and essentially zero meat protein in the dry).
Alternative treatments such as herbal combinations, acupuncture, etc. can help many things, but feline hyperthyroidism is not one of them. Approaches such as Thyroid Support Gold may reduce visible symptoms, but they do not lower the T4. The body continues to run at high speed, and the cat is actually still getting sicker while the problem is hidden. It's possible that such treatments help in thyroid issues not caused by a tumor, but use to treat feline hyperthyroidism is problematic at best.
The primary advances have been in improving dosing protocols with methimazole and tailoring the dose of I-131. Stays at 131 clinics that were once 2 weeks or more are now often just 3-5 days in the US. Most clinics outside of the US are lagging behind in this regard but are starting to catch up. However, there remain issues with the quality of information and how information circulates among vets.
We currently have quite a few fur-kin that we care for or help care for. All were strays at one time, and some have extra issues with one asthmatic, one FIV positive, one FeLV positive, and one hyperthyroid. The information and stories are free to all, and that especially applies to the medical material since I don't want to be accused of practicing without a license, but donations are certainly appreciated.