Heart murmur after Levothyroxine in dogs

The FDA has received 27 reports of heart murmur in dogs after Levothyroxine. That is 0.9% of all reports for this medicine, compared with 0.2% across reports for all dog medicines, so it is reported unusually often with this medicine.

27Reports

78%Marked serious

11%Recovered

15%Died or euthanised

100 daysTypical onset

How soon it started

Same day

0 0.0%

1–2 days

0 0.0%

3–7 days

0 0.0%

1–4 weeks

2 14%

Over a month

12 86%

How the animals did

Recovered

1 3.7%

Recovered with lasting effects

2 7.4%

Still unwell when reported

12 44%

Died

2 7.4%

Euthanised

2 7.4%

Not known

8 30%

Problems reported at the same time

11 41%

11 41%

8 30%

8 30%

8 30%

Abnormal radiograph finding

6 22%

Weight loss

5 19%

Death

4 15%

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 heart murmur in dogs?: 27 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 100 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).

Heart murmur with similar medicines

Heart murmur after Trilostane (Vetoryl) 159 reports · 1.3% of its reports
Heart murmur after Insulin 47 reports · 0.6% of its reports
Heart murmur after Phenylpropanolamine (Proin) 23 reports · 0.6% of its reports

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