Is Thyroid Medications Toxic to Cats and Dogs?
Thyroid medications are moderately toxic to both cats and dogs — levothyroxine overdose speeds up the heart and metabolism; methimazole overdose causes stomach and blood effects.
Can cause real harm — call a vet for guidance.
Can cause real harm — call a vet for guidance.
In an emergency, do not wait.
Medical disclaimer: This tool is for general information only and isnot a substitute for veterinary diagnosis or treatment. Toxicity depends on the amount, your pet’s size, and health history. If your pet may have ingested something harmful, contact your veterinarian or a poison control hotline ((888) 426-4435 / (855) 764-7661) immediately — do not wait for symptoms to appear.
What is Thyroid Medications?
Thyroid medications are moderately toxic to both cats and dogs — levothyroxine overdose speeds up the heart and metabolism; methimazole overdose causes stomach and blood effects.
Also known as: levothyroxine, synthroid, thyroxine, euthyrox, methimazole, tapazole, thyroid tablets, armour thyroid, hypothyroid meds, hyperthyroid meds.
Symptoms to Watch For
🐕 In Dogs
- Restlessness, hyperactivity, and panting within hours to a day
- Rapid heartbeat and rapid breathing
- Vomiting, diarrhea, and increased thirst
- Tremors and fever in large overdoses
🐈 In Cats
- Vomiting and reduced appetite (more typical of methimazole overdose)
- Lethargy and drooling
- Increased thirst or changes in breathing
- Bruising or bleeding with large methimazole overdoses
What to Do If Ingested
- Remove any remaining tablets from reach and count how many are missing.
- Note the dose (mcg) and your dog's weight — dose math matters here.
- Offer water and keep the dog calm.
- Call a vet or poison hotline for guidance, even if no symptoms have started.
Steps apply to both cats and dogs. When in doubt, call a vet or poison hotline before acting. Have a cat? Run the Pet Poison Triage Wizard for cat-specific steps.
When to Call the Vet Immediately
Call your veterinarian or a poison control hotline right away if you notice any of these:
- More than a few extra tablets were eaten (the threshold depends on the dose)
- Hyperactivity, panting, or a racing heart
- Tremors, vomiting, or fever
Frequently Asked Questions
Frequently Asked Questions
My dog just ate my levothyroxine. Dogs take that drug too — is it actually dangerous?
Dogs are indeed prescribed levothyroxine, so an extra tablet or two is usually minor — but the human doses vary widely, and a small dog eating a large number of high-strength tablets can develop a real thyroid storm: racing heart, hyperactivity, panting, tremors, and fever. Call a vet or poison hotline with the tablet strength (mcg), the count eaten, and your dog's weight for dose-specific advice.
What is the difference between levothyroxine and methimazole exposures?
They treat opposite problems. Levothyroxine replaces thyroid hormone — an overdose speeds the metabolism up (racing heart, hyperactivity). Methimazole suppresses hormone production — an overdose slows things down, causing vomiting, drooling, and in large overdoses blood-cell suppression with bruising or bleeding. Both warrant a vet call; note which one was eaten because the treatment differs.
Which pets are most at risk from thyroid pill bottles?
Dogs that raid pill organizers and handbags are the classic case — levothyroxine is one of the most commonly prescribed medications and often sits in a weekly pill minder that smells faintly of the owner. Cats rarely steal tablets, but a cat being treated for hyperthyroidism can get a double dose if the household loses track of who was medicated. Keep pill organizers closed and stored up high.
Sources
This entry is reviewed against the following authoritative veterinary sources.
Last reviewed: 2026-10-16