Lethargy after Levothyroxine in dogs

470 reports to the FDA describe lethargy in dogs given Levothyroxine. That is 16% of all reports for this medicine, compared with 11% across reports for all dog medicines.

470Reports

54%Marked serious

35%Recovered

16%Died or euthanised

53 daysTypical onset

How soon it started

Same day

41 14%

1–2 days

25 8.3%

3–7 days

26 8.6%

1–4 weeks

44 15%

Over a month

166 55%

How the animals did

Recovered

144 31%

Recovered with lasting effects

22 4.7%

Still unwell when reported

119 25%

Died

21 4.5%

Euthanised

55 12%

Not known

109 23%

Problems reported at the same time

223 47%

175 37%

97 21%

75 16%

75 16%

66 14%

47 10%

47 10%

Age of the animals

Under 1 year

1–3 years

4–7 years

117

8–11 years

194

12+ years

147

About this medicine

Levothyroxine treats an underactive thyroid.

Questions owners ask

Can Levothyroxine cause lethargy in dogs?: 470 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 53 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).

Lethargy with similar medicines

Lethargy after Trilostane (Vetoryl) 2,568 reports · 22% of its reports
Lethargy after Insulin 895 reports · 12% of its reports
Lethargy after Phenylpropanolamine (Proin) 668 reports · 16% of its reports

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