Behaviour change after Levothyroxine in dogs

The FDA has received 117 reports of behaviour change in dogs after Levothyroxine. That is 3.9% of all reports for this medicine, compared with 2.5% across reports for all dog medicines, so it is reported unusually often with this medicine.

117Reports

64%Marked serious

30%Recovered

16%Died or euthanised

37 daysTypical onset

How soon it started

Same day

14 21%

1–2 days

4 5.9%

3–7 days

5 7.4%

1–4 weeks

10 15%

Over a month

35 51%

How the animals did

Recovered

31 26%

Recovered with lasting effects

4 3.4%

Still unwell when reported

36 31%

Died

4 3.4%

Euthanised

15 13%

Not known

27 23%

Problems reported at the same time

32 27%

26 22%

23 20%

19 16%

18 15%

16 14%

14 12%

Restlessness or agitation

14 12%

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 behaviour change in dogs?: 117 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 37 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).

Behaviour change with similar medicines

Behaviour change after Trilostane (Vetoryl) 445 reports · 3.7% of its reports
Behaviour change after Insulin 189 reports · 2.6% of its reports
Behaviour change after Phenylpropanolamine (Proin) 210 reports · 5.1% of its reports

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