Is Levothyroxine (Synthroid) Safe for Dogs?
Why it’s a problem
Levothyroxine is thyroid hormone, and dogs that eat a large number of tablets have been reported to develop hyperthyroid signs: racing heart, panting, hyperactivity, and increased thirst. A single tablet often causes little more than mild stomach upset, but the risk grows with the number eaten and the smaller the dog.
How to keep your pet safe
Keep thyroid pills in closed storage and count doses if you use a pill organizer. Thyroid conditions in dogs should be diagnosed and dosed by a veterinarian, not self-treated with human tablets.
Warning signs to watch for
Vomiting, diarrhea, restlessness, panting, rapid heart rate, increased thirst and urination, hyperactivity; large ingestions: tremors, fever, rapid breathing.
When to call the vet
Call your veterinarian or a pet poison hotline with the tablet strength and the number possibly eaten — risk depends heavily on dose and body weight. Do not wait for symptoms if a large number of pills was involved.
Common questions
My dog ate my Synthroid — is it an emergency?
One tablet in a larger dog is usually mild, but several tablets — or any amount in a small dog — can cause real thyroid toxicity. Call your vet or a poison hotline for a dose-based assessment.
Can I treat my dog’s hypothyroidism with my levothyroxine?
No. Dogs need veterinary-diagnosed dosing of pet-appropriate products. Never self-dose — thyroid excess strains the heart.
More medications hazards for dogs
This page is general educational information, not a substitute for professional veterinary advice. Every pet is different — if your animal ate or touched something concerning, contact your veterinarian or a pet poison hotline right away. In the US: Pet Poison Helpline (855) 764-7661, ASPCA Animal Poison Control (888) 426-4435 (fees may apply).