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Methimazole dose for hyperthyroid cats, adjusted by T4
Methimazole starts at 1.25 to 2.5 mg per cat every 12 hours by mouth or on the ear, and the Vetinuity methimazole dose table below changes it in 1.25 to 2.5 mg steps until total T4 sits in the lower half of the reference interval, with a CBC, chemistry and T4 every 2 to 3 weeks for the first 3 months.
Methimazole doses at a glance
| Species | Use | Dose | Route and interval |
|---|---|---|---|
| Cat | Starting dose, oral | 1.25 to 2.5 mg per cat | PO every 12 hours |
| Cat | Starting dose, transdermal gel | 2.5 mg per cat | Inner pinna every 12 hours |
| Cat | Dose change after a T4 recheck | 1.25 to 2.5 mg per day | Up or down, then recheck in 2 to 3 weeks |
| Dog | Not a routine canine drug | No standard dose given here | Canine hyperthyroidism is usually a thyroid tumor |
Vetinuity methimazole dose calculator
Starting low
The 2016 AAFP guidelines favor a low starting dose given twice daily, because lower doses cause fewer adverse effects and still bring most cats into range within weeks1. Transdermal methimazole in a compounded gel causes fewer gastrointestinal effects than tablets but tends to work more slowly, so the first recheck can show a T4 that is falling but not yet in range1.
Reading T4 on treatment
The target is a total T4 in the lower half of the laboratory's reference interval. A T4 above the interval at the 2 to 3 week recheck earns a dose increase of 1.25 to 2.5 mg per day; a T4 below the interval, especially in an azotemic cat, earns a decrease, because iatrogenic hypothyroidism in a cat with kidney disease is linked to worse kidney function1. Timing of the sample relative to the pill matters less than consistency from one recheck to the next.
Kidney values unmasked
Hyperthyroidism raises the glomerular filtration rate, which hides chronic kidney disease. As T4 falls, creatinine and SDMA often rise, so both are measured beside every T4 in the first months. A cat whose creatinine climbs sharply as the T4 normalizes may be better left with a T4 in the upper half of the interval1.
Blood counts and liver values
In the original series of 262 cats, the serious reactions, agranulocytosis, thrombocytopenia and hepatopathy, almost all appeared within the first 3 months of treatment2. That is why the CBC and liver panel ride along with every early recheck. Any of those reactions means the drug is stopped for good, not reduced, and the cat moves to radioiodine, surgery or diet.
Related values on this site: Creatinine, SDMA, Platelet count, ALT.
Frequently asked questions
What T4 should a cat on methimazole have?
A total T4 in the lower half of the laboratory's reference interval, without azotemia developing as it falls1.
How often are rechecks needed at the start?
Every 2 to 3 weeks for the first 3 months, with a CBC and chemistry beside the T4, because the serious adverse reactions cluster in that window2,1.
Is transdermal methimazole as effective as tablets?
It works but tends to be slower, with fewer gastrointestinal side effects1.
Sources
- Carney HC, Ward CR, Bailey SJ, et al. 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. J Feline Med Surg. 2016; 18(5):400-16. PMID: 27143042. DOI: 10.1177/1098612X16643252.
- Peterson ME, Kintzer PP, et al. Methimazole treatment of 262 cats with hyperthyroidism. J Vet Intern Med. 1988; 2(3):150-7. PMID: 3265728. DOI: 10.1111/j.1939-1676.1988.tb02812.x.