Your cat is thirteen, eating like she's starving, and losing weight anyway. Maybe she's drinking more water than she used to, or yowling at 3:00 AM for no reason you can find, or her coat looks rough in a way it didn't six months ago. The vet ran a senior panel, and now there's a number on the printout called T4, total thyroxine, and it's flagged high. The DVM says it's hyperthyroidism and recommends a medication called methimazole. You're looking at a twice-daily pill for the rest of her life.
That's the conversation most owners walk away from wanting to understand better. What the thyroid is actually doing, what the medication controls, what the side effects look like in practice, and the part that rarely comes up at the first appointment: what treating the thyroid can reveal about the kidneys.
What hyperthyroidism is and why it's almost always cats over ten
Hyperthyroidism is the most common endocrine disease in older cats. The Merck Veterinary Manual, the reference textbook clinicians use, describes it as a sustained overproduction of thyroid hormone driven by a benign growth called a thyroid adenoma. In almost every case, the adenoma is non-cancerous. It just produces more T4 than the body needs, and excess T4 speeds up metabolism across every organ system.
The reason it shows up almost exclusively in cats over ten is still not fully understood. The Merck Vet Manual notes that environmental factors, dietary iodine, and certain flame retardants have all been studied as potential contributors, but no single cause has been confirmed. What is confirmed is the clinical picture: a senior cat who is eating well and losing weight, often with a faster heart rate, increased thirst and urination, occasional vomiting, restlessness or night vocalization, and a coat that looks unkempt. Not every hyperthyroid cat shows every symptom. The weight loss despite a strong appetite is the one that most reliably points the vet toward checking the T4.
How the diagnosis works
A total T4 blood test is the first step, and in most cases it's enough. A high T4 in a cat with the classic symptoms confirms the diagnosis. The walk-through of what each value on a cat's bloodwork panel means covers where T4 sits on the printout and how the vet reads it against the rest of the chemistry.
One wrinkle matters. A sick cat with another illness can have a T4 that reads falsely normal, because the other illness suppresses thyroid hormone levels. When a DVM strongly suspects hyperthyroidism but the total T4 is in range, the next step is usually a free T4 test or a repeat test in a few weeks. A thyroid scan, called a technetium scintigraphy, is less common and reserved for cases where the vet needs to see whether one or both lobes are overactive, usually before considering surgery or radioactive iodine.
Four treatment paths, honestly compared
Methimazole (or Felimazole). The most common first-line treatment. Methimazole blocks the thyroid gland from producing excess T4. It comes in tablets given once or twice daily, or as a transdermal gel applied to the inner ear. Plumb's Veterinary Drug Handbook, the dosing reference most US vets keep on their desk, lists typical starting doses of 1.25 to 2.5 mg twice daily, with the number adjusted at rechecks. The dose your vet sets is the one that matters for your cat. Methimazole isn't a cure. It controls the hormone for as long as the cat takes it, and stopping the medication means the T4 climbs back. The advantages are accessibility, low cost relative to the alternatives, and no sedation required to start. Bloodwork monitoring every three to six months once stable is part of the deal.
Radioactive iodine (I-131). The treatment most internists consider the gold standard. A single injection of radioactive iodine destroys the overactive thyroid tissue while leaving normal tissue intact. It cures the disease in over 95% of cases. The cat stays at a specialty facility for two to five days while the radiation clears. Cost runs $1,500 to $2,500 depending on the facility and region, and not every area has a center that offers it. For a cat with no other significant health issues, I-131 eliminates the need for lifelong medication and monitoring.
Surgery (thyroidectomy). Surgical removal of the affected thyroid lobe is curative when successful. In practice, it's chosen less often than methimazole or I-131 because it requires general anesthesia in a senior cat, which carries higher risk, and because the parathyroid glands sit close to the thyroid and can be damaged during the procedure, leading to dangerous drops in calcium. Surgery makes sense in specific cases, but most DVMs discuss it as a second or third option.
Prescription diet (Hill's y/d). An iodine-restricted diet that limits the raw material the thyroid needs to produce hormone. It works if the cat eats nothing else, which means no treats, no other food, no hunting. In a multi-cat household, strict diet separation is difficult. In a single-cat household with an indoor cat who will eat the food, it's a real option worth discussing. The vet conversation is where the specifics of your cat tip the decision.
The part most owners aren't told first: thyroid treatment can unmask kidney disease
This is the section that earns the visit. On the internal-medicine floor at Angell, I watched this pattern play out enough times that it stopped surprising me. A hyperthyroid cat comes in with a creatinine of 1.6, which looks fine. The vet starts methimazole. Three weeks later, at the first recheck, the T4 has dropped into range and the creatinine has climbed to 2.8. The kidneys didn't get worse in three weeks. They were always there. The thyroid was hiding them.
Excess thyroid hormone increases blood flow to the kidneys, which pushes creatinine and BUN, the two main kidney markers, artificially low. The kidneys look better than they are because the thyroid is running the blood through them at higher pressure and volume. Once methimazole brings the T4 down, the kidneys lose that boost and the true baseline appears. BUN is blood urea nitrogen, a number that tracks how the kidneys clear waste. Creatinine is the kidney number most DVMs watch over time. Both are explained value by value in the kidney disease staging guide.
This is why the vet checks kidney values at every methimazole recheck, not just at the start. It's also why some cats end up on a lower methimazole dose than what would fully normalize the T4. The DVM is balancing two conditions against each other: bring the thyroid hormone down enough to protect the heart, but not so far that the kidneys can't keep up. That balance is specific to your cat and changes over time. The recheck is where the numbers tell the vet whether the balance is holding.
What to track at home between rechecks
Vets read the bloodwork. You read the cat. The four things worth tracking between rechecks, in the order most internists would list them:
Weight, weekly. A kitchen scale works. Weigh the cat in a carrier or a box, subtract the container, write it down. Weight loss that continues after treatment starts means the dose may need adjustment. Weight gain once treatment stabilizes is normal and expected.
Appetite changes. A hyperthyroid cat eats ravenously. Once the T4 normalizes, appetite usually settles to something closer to normal, and that's fine. A cat who stops eating after starting methimazole is a different signal. If your cat skips meals for more than a day, the appetite guide covers why the 24-hour window matters in cats and when it's a call to the clinic.
Water intake. Increased thirst is one of the classic signs. If it doesn't decrease after treatment, the vet wants to know, because persistent polydipsia after the T4 is controlled may point toward kidney function rather than the thyroid.
Energy level. A hyperthyroid cat is often restless, vocal, wired. After treatment, a normal slowdown is fine. A cat who becomes profoundly lethargic or withdrawn is showing something the vet should see.
Keeping a simple log of those four things, even a note on your phone with the date and a few words, is the kind of information that makes the recheck conversation faster and more useful. The medication tracking guide covers the system side of keeping a multi-medication household organized.
Methimazole side effects in practice
Most cats tolerate methimazole well. The ones who don't tend to show it in the first two to four weeks. Appetite loss, vomiting, and mild lethargy are the most common early reactions, and in many cases the vet can adjust the dose or switch from oral to transdermal to resolve them. Plumb's lists the full side-effect profile, and the range is wide, but the practical takeaway is this: if your cat stops eating or starts vomiting after beginning the medication, call the clinic. Don't stop the medication on your own and don't wait a week to see if it passes.
Facial itching, scratching around the ears and head, shows up in a small percentage of cats and is a known dermatologic reaction. It usually means the medication itself needs to change, not just the dose.
The serious side effects are rarer and are what the recheck bloodwork specifically screens for: liver enzyme elevation and drops in white blood cell counts or platelet counts. Neither is common, but both are why the vet draws blood at scheduled intervals rather than relying on how the cat looks at home. The dose your vet set is the dose for your cat. If the number on your bottle doesn't match what you expected, the bottle wins. Call the clinic and ask.
Questions that come up after the diagnosis
How common is hyperthyroidism in cats?
It’s the most frequently diagnosed endocrine disease in cats. The Merck Veterinary Manual puts the typical onset at ten years and older, with some cats diagnosed as early as eight. By the time a cat is fifteen, the odds that a thyroid adenoma has started producing excess hormone are high enough that most senior panels include a T4 by default.
Can methimazole cure hyperthyroidism?
No. Methimazole controls thyroid hormone production for as long as the cat takes it, but it doesn’t eliminate the adenoma that’s producing it. Stop the medication and the T4 climbs back up. The only curative treatments are radioactive iodine (I-131), which destroys the overactive tissue, and surgical thyroidectomy, which removes it. Methimazole is the most common choice because it’s the most accessible and the least invasive to start, especially in a senior cat where sedation risk matters.
What does methimazole cost per month?
Retail pricing for generic methimazole tablets runs roughly $15 to $40 per month at most US pharmacies. Transdermal methimazole compounded into an ear gel, which some owners prefer for cats who won’t take pills, typically costs $30 to $60 per month through a compounding pharmacy. The medication cost is the smaller part of the ongoing expense. The rechecks, bloodwork every three to six months once stable, add more to the annual total than the pills do.
Why does my vet keep checking kidney values after starting thyroid medication?
Because hyperthyroidism masks kidney disease. Excess thyroid hormone increases blood flow to the kidneys, which makes creatinine and BUN look better than they actually are. Once methimazole brings the T4 down, the kidneys lose that artificial boost and the true kidney function becomes visible. A cat whose creatinine was 1.8 before treatment might settle at 2.5 once the thyroid is controlled, and 2.5 changes the staging conversation. The vet checks kidney values at every recheck specifically to catch that shift early.
Is the transdermal ear gel as effective as the pill?
For most cats, yes. Studies published in the Journal of Veterinary Internal Medicine have shown comparable T4 control between oral methimazole and the transdermal formulation. Absorption is slightly less predictable with the gel, so the vet may need an extra recheck early on to confirm the dose is landing. Most owners who switch to transdermal do it because the twice-daily pilling was turning into a fight, and the tradeoff in absorption variability is worth the household peace.
Should I consider radioactive iodine instead of methimazole?
If your cat is otherwise healthy and you can manage the logistics, I-131 is the treatment most internists consider the gold standard. One treatment, usually a two- to five-day hospital stay at a specialty facility, and the overactive thyroid tissue is gone. The cat comes home cured. The catch is availability and cost. Not every region has a facility that offers it, and the treatment runs $1,500 to $2,500. For a twelve-year-old cat with early CKD and a heart murmur, the math looks different than for a ten-year-old cat with nothing else going on. The vet conversation is where the specifics of your cat tip the decision.
What side effects should I watch for on methimazole?
The most common ones show up in the first few weeks: appetite loss, vomiting, and lethargy. Most cats who are going to have GI side effects show them early, and the vet can adjust the dose or switch to transdermal. Rarer but documented: facial itching or scratching around the ears and head, which shows up in a small percentage of cats and usually means the medication needs to change. The serious ones, liver enzyme elevation or drops in white blood cell counts, are what the recheck bloodwork is looking for. Plumb’s Veterinary Drug Handbook lists the full profile. If your cat stops eating after starting methimazole, that’s a same-day call to the clinic, not a wait-and-see.
The recheck is the treatment
A hyperthyroid cat on methimazole isn't managed by the pill. The pill is the easy part. The management is the recheck: T4 to confirm the dose is working, kidney values to see what the thyroid was hiding, a CBC to confirm the medication isn't suppressing the blood cells. The three- to six-month rhythm of those rechecks, combined with what you track at home between them, is how a thirteen-year-old cat with an overactive thyroid lives well for years. Bring the prior panel to the next visit. The trend is where the medicine actually happens.
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Rachel Howland, CVT (ret.), spent a decade in clinic: seven years in a mixed practice in upstate New York, then three on the internal-medicine floor at Angell Animal Medical Center in Boston. She left practice in 2017 and has written about small-animal health since. She lives in Somerville with Juno, an 11-year-old hound mix managing chronic kidney disease, and Bishop, a Siamese cat. She does not diagnose or prescribe; she explains what your vet's records are telling you and what questions are fair to ask.