Methimazole is cheap, effective and the most common first treatment for an overactive thyroid in a cat, and the first 3 months on it are a fixed schedule rather than a matter of judgement. Blood is drawn at day 14, then every 3 weeks 3 times over, at day 35, 56 and 77, and after 3 months the cat moves to long-term monitoring. Between 5 and 15 percent of cats develop a side effect that shows up in that window, which is exactly why the window exists.
Why day 14 cannot slip
Two of the serious reactions to methimazole, a fall in white cells and liver injury, usually appear between day 7 and day 56, and both are reversible if the drug stops early. A cat can look completely normal with a neutrophil count on its way down. The reference range for neutrophils is roughly 2.5 to 12.5 x 10^9/L, and most vets stop the drug below 1.5 x 10^9/L or at any fever with a falling count. ALT has a reference of 10 to 100 U/L, and a value more than 3 times the top of the range, so over 300 U/L, is the usual stopping line. Waiting for symptoms means waiting 3 to 7 days too long in a cat that was fine at breakfast. The day 0 panel is what makes the day 14 result readable, so the baseline numbers are worth keeping in the cat's file.
The schedule in dates
| Visit | Day | Tests | Decision point |
|---|---|---|---|
| Baseline | day 0 | T4, CBC, chemistry, blood pressure, weight | confirm diagnosis, set starting dose |
| First recheck | day 14 | T4, CBC, ALT | stop if neutrophils under 1.5 |
| Second | day 35 | T4, CBC, ALT | adjust if T4 above 2.5 ug/dL |
| Third | day 56 | T4, ALT, weight | watch creatinine rise |
| Fourth | day 77 | T4, CBC, ALT, blood pressure | confirm stability |
| Month 3 review | day 90 to 105 | T4, creatinine, SDMA, blood pressure | move to long-term plan |
The 3 weekly checks after day 14 exist because most dose changes happen in the first 8 weeks, and each change needs its own 2 to 3 week read. A cat that reaches day 77 with a stable T4 and normal liver values has cleared the risky period, and the interval can stretch to every 3 months and then every 6.
Reading the numbers at each visit
Total T4 has a reference of 1.0 to 4.0 ug/dL, and the treatment target is roughly 1.0 to 2.5 ug/dL, in the lower half of normal rather than at the bottom. A T4 under 1.0 ug/dL means over-treatment, which drops the heart rate and the kidney filtration rate together. Creatinine has a reference of 0.9 to 2.5 mg/dL, and a rise of more than 0.5 mg/dL in the first 3 months suggests kidney disease that the hyperthyroidism was hiding: the high thyroid hormone was propping up filtration, and 20 to 30 percent of cats show this once the thyroid is controlled. Different laboratories set slightly different ranges, so a trend on the same analyzer beats a single value against a printed number.
Side effects: what the 5 to 15 percent looks like
- Vomiting. Mild and once or twice in the first 2 weeks is common and often settles; 3 or more times in 24 hours, or vomiting with a cat that stops eating, means stop and call.
- Itching and facial sores. Pruritus around the head and neck, and excoriation at the ear tips, are the classic skin reaction, usually between day 10 and day 30.
- Lethargy and jaundice. Yellow gums or ear pinnae means liver involvement; stop the drug and call the same day, because this one does not settle on its own.
- Low white cells. Under 1 percent of cats develop agranulocytosis, and it presents as fever and mouth ulcers rather than as itching.
A cat that vomits once and eats breakfast the next morning is not the same as a cat that refuses 2 meals. The second cat needs bloodwork, not a wait-and-see week.
After 3 months, the long-term rhythm
Stable cats move to a T4 check every 3 months in the first year, then every 6 months, with a full panel and blood pressure once a year. Weight is part of the check: most cats regain 0.3 to 0.5 kg in the first 3 to 6 months, and a cat that gains nothing has either an under-treated thyroid or another problem. A cat that gains 1 kg in 3 months on treatment needs a second look, because over-treatment slows metabolism and can mask a rising creatinine.
Dose, formulation and the mistakes to avoid
The tablet is small and bitter, and the dose is set by the vet by weight and kidney function, usually given once or twice daily, never split by eye or adjusted at home. A missed dose is skipped, not doubled. Transdermal gel into the inner ear is an option for a cat that cannot be pilled, at a dose the vet sets the same way, and it needs the same blood schedule. An iodine-restricted diet is the other route and should not be mixed with tablets unless the vet plans it, because the two together can push T4 too low; the choice between tablets, diet and radioiodine turns on the cat's kidney values and temperament.
Cost runs modest for the monitoring: T4 30 to 60 USD, a CBC 40 to 80 USD, a full panel 60 to 150 USD, a visit 50 to 100 USD, and the medication 15 to 40 USD a month. The first 3 months with 4 blood draws land around 300 to 600 USD, and that is the part owners underestimate when they choose tablets over radioiodine, as the cost breakdown shows.
When to stop the drug and call
Jaundice, vomiting 3 or more times in 24 hours, refusal to eat for 24 hours, fever, mouth ulcers, or a face that suddenly itches and bleeds all mean stop the tablet and phone the clinic. Do not restart on your own the next day: the vet needs the blood result first, and in most of these cases the plan changes to a different drug or to surgery.
- Put day 14 and day 35 in the calendar before the first tablet goes in.
- Ask for T4, CBC and ALT at every visit for the first 8 weeks.
- Weigh the cat on the same scale at each visit and write the number down.
- Keep the clinic number visible, and stop the drug for jaundice or 3 vomits in 24 hours.
- At month 3, ask what the long-term interval will be and book the next 2 dates.
Terms mentioned in this article
Frequently asked
Why does my cat need blood tests 2 weeks after starting methimazole?
Because the two serious reactions, a falling white cell count and liver injury, usually appear between day 7 and day 56 and are reversible if the drug stops early. A cat can look normal with neutrophils heading below 1.5 x 10^9/L or an ALT climbing past 300 U/L. Waiting for vomiting or jaundice adds 3 to 7 days to a problem that a blood draw catches first.
How long until the monitoring gets easier?
Day 14, then every 3 weeks three times, at day 35, 56 and 77, then a review at month 3. A cat that is stable at day 77 has cleared the risky window and moves to a T4 check every 3 months for the first year, then every 6 months with a full panel and blood pressure once a year.
What side effects should make me stop the tablet?
Jaundice, vomiting 3 or more times in 24 hours, refusal to eat for 24 hours, fever, mouth ulcers, or a face that suddenly itches and bleeds. Between 5 and 15 percent of cats have some reaction, most often mild vomiting or itching in the first 2 to 4 weeks. Stop the drug and call the clinic rather than restarting it the next day, because the vet needs the blood result first.