Monocytosis after Levothyroxine in dogs

26 reports to the FDA describe monocytosis in dogs given Levothyroxine. That is 0.9% of all reports for this medicine, compared with 0.3% across reports for all dog medicines, so it is reported unusually often with this medicine.

26Reports

85%Marked serious

7.7%Recovered

46%Died or euthanised

13 daysTypical onset

How soon it started

Same day

1 5.9%

1–2 days

3 18%

3–7 days

1 5.9%

1–4 weeks

5 29%

Over a month

7 41%

How the animals did

Recovered

1 3.8%

Recovered with lasting effects

1 3.8%

Still unwell when reported

8 31%

Died

3 12%

Euthanised

9 35%

Not known

4 15%

Problems reported at the same time

17 65%

15 58%

14 54%

13 50%

12 46%

12 46%

12 46%

11 42%

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 monocytosis in dogs?: 26 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 13 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).

Monocytosis with similar medicines

Monocytosis after Trilostane (Vetoryl) 83 reports · 0.7% of its reports
Monocytosis after Insulin 24 reports · 0.3% of its reports

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