Hypothyroidism after Levothyroxine in dogs

66 reports to the FDA describe hypothyroidism in dogs given Levothyroxine. That is 2.2% 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.

66Reports

68%Marked serious

11%Recovered

17%Died or euthanised

29 daysTypical onset

How soon it started

Same day

7 21%

1–2 days

1 2.9%

3–7 days

6 18%

1–4 weeks

5 15%

Over a month

15 44%

How the animals did

Recovered

2 3.0%

Recovered with lasting effects

5 7.6%

Still unwell when reported

34 52%

Died

3 4.5%

Euthanised

8 12%

Not known

14 21%

Problems reported at the same time

22 33%

15 23%

15 23%

14 21%

Didn't work as expected

13 20%

Blood problems

11 17%

Death

11 17%

Excessive thirst

11 17%

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 hypothyroidism in dogs?: 66 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 29 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).

Hypothyroidism with similar medicines

Hypothyroidism after Trilostane (Vetoryl) 96 reports · 0.8% of its reports
Hypothyroidism after Insulin 37 reports · 0.5% of its reports

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