Hypothyroidism after Methimazole in cats

26 reports to the FDA describe hypothyroidism in cats given Methimazole. That is 1.5% of all reports for this medicine, compared with 0.1% across reports for all cat medicines, so it is reported unusually often with this medicine.

26Reports

54%Marked serious

23%Recovered

12%Died or euthanised

31 daysTypical onset

How soon it started

Same day

0 0.0%

1–2 days

0 0.0%

3–7 days

2 9.5%

1–4 weeks

8 38%

Over a month

11 52%

How the animals did

Recovered

6 23%

Still unwell when reported

3 12%

Died

1 3.8%

Euthanised

2 7.7%

Not known

14 54%

Problems reported at the same time

14 54%

9 35%

8 31%

6 23%

4 15%

4 15%

4 15%

3 12%

Age of the animals

Under 1 year

1–3 years

4–7 years

8–11 years

12+ years

About this medicine

Methimazole treats an overactive thyroid in cats.

Questions owners ask

Can Methimazole cause hypothyroidism in cats?: 26 reports link the two, but a report alone does not prove the medicine was the cause: other illnesses, other medicines or coincidence can be involved. Your vet can judge it for your animal.
How soon after a dose was it reported?: In half of the reports with dates, it started within 31 days of the first dose.
What should I do?: Don't give another dose until you've spoken to your vet, and keep the packaging so you can tell them the product and dose. Your vet can also report it to the FDA (or you can, through the manufacturer or FDA form 1932a).

All reported side effects of Methimazole in cats · Medicines most reported with hypothyroidism