What are the symptoms of Thyroid medication poisoning in cats?

vomiting, restlessness, abnormal heartbeat

Thyroid medication

Q: What if my cat ate Thyroid medication?

Toxic: Toxic in overdose — too much thyroid hormone or methimazole is a vet problem, not a home adjustment. Levothyroxine (Synthroid) and methimazole (Tapazole) are both used in cats — under veterinary direction. A spilled human bottle or a double-dose mix-up causes vomiting and heart-rate changes. This page does not tell you how to restart a missed dose. Source: ASPCA.

Q: Is Blood-pressure medication also safe for cats?

Toxic: Toxic in overdose — the cat can become profoundly weak or collapse.

Toxic

Verdict

Toxic in overdose — too much thyroid hormone or methimazole is a vet problem, not a home adjustment.

Toxic principle: levothyroxine or methimazole overdose

Also called: synthroid, levothyroxine, methimazole, tapazole

Source: ASPCA · Verified 2026-08-13

If your cat ate this

Cats hide symptoms — don't wait and see. Even small amounts of certain substances can be fatal to a cat. Act fast.

1. Call your veterinarian immediately
US — National hotline: ASPCA Animal Poison Control Center (888) 426-4435 aspca.org(24/7, Consultation fee may apply.)
US — National hotline: Pet Poison Helpline (855) 764-7661 petpoisonhelpline.com(24/7, $89 per incident fee; also covers Canada.)
GB — National hotline: Animal PoisonLine 01202 509 000 animalpoisonline.co.uk(24/7, £35 per call (8am–8pm weekdays), £45 at other times.)
AU — National hotline: Animal Poisons Helpline 1300 869 738 animalpoisons.com.au(24/7)
NZ — National hotline: Animal Poisons Helpline 0800 869 738 poisons.co.nz(11am–7pm NZ time (currently being extended), Charges apply.)
CA — National hotline: Pet Poison Helpline (855) 764-7661 petpoisonhelpline.com(24/7, $89 per incident fee.)

Verified 2026-08-06

Verdict

Levothyroxine (Synthroid) and methimazole (Tapazole) are both used in cats — under veterinary direction. A spilled human bottle or a double-dose mix-up causes vomiting and heart-rate changes. This page does not tell you how to restart a missed dose.

Symptoms to watch for

• vomiting
• restlessness
• abnormal heartbeat

What to do

Call the prescribing vet or poison control with the tablet strength and count.

ASPCA Animal Poison Control Center: (888) 426-4435 aspca.org

Related foods

Blood-pressure medication

Toxic

Toxic in overdose — the cat can become profoundly weak or collapse.

Prescription

ADHD and stimulant medicines

Dangerous

Dangerous — amphetamines and related stimulants cause life-threatening agitation.

Prescription

SSRI antidepressants

Toxic

Toxic in overdose — serotonin syndrome is an emergency. Do not start these at home.

Prescription

Birth-control pills

Caution

Caution — a few pills usually cause stomach upset, not a hormone crisis. Still call if a pack is missing.

Prescription