Seizures after Levothyroxine in dogs

206 reports to the FDA describe seizures in dogs given Levothyroxine. That is 6.8% of all reports for this medicine, compared with 4.4% across reports for all dog medicines, so it is reported unusually often with this medicine.

206Reports

86%Marked serious

40%Recovered

17%Died or euthanised

28 daysTypical onset

How soon it started

Same day

16 13%

1–2 days

18 14%

3–7 days

11 8.6%

1–4 weeks

23 18%

Over a month

60 47%

How the animals did

Recovered

77 37%

Recovered with lasting effects

6 2.9%

Still unwell when reported

55 27%

Died

9 4.4%

Euthanised

26 13%

Not known

33 16%

Problems reported at the same time

35 17%

26 13%

25 12%

22 11%

21 10%

19 9.2%

19 9.2%

12 5.8%

Age of the animals

Under 1 year

1–3 years

4–7 years

8–11 years

12+ years

About this medicine

Levothyroxine treats an underactive thyroid.

Questions owners ask

Can Levothyroxine cause seizures in dogs?: 206 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 28 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).

Seizures with similar medicines

Seizures after Trilostane (Vetoryl) 367 reports · 3.1% of its reports
Seizures after Insulin 396 reports · 5.4% of its reports
Seizures after Phenylpropanolamine (Proin) 195 reports · 4.7% of its reports

All reported side effects of Levothyroxine in dogs · Medicines most reported with seizures