Is Levothyroxine (Synthroid) Safe for Cats?
Why it’s a problem
Cats have a small body size, so even a few human tablets can push thyroid hormone levels dangerously high. Exposures have been reported to cause racing heart rate, hyperactivity, panting, and vomiting. Cats with heart conditions are at particular risk from thyroid overstimulation.
How to keep your pet safe
Store levothyroxine in a cabinet cats cannot open, and pick up any dropped pills right away. If you suspect your cat has a thyroid problem, see a veterinarian — human tablets are not a safe home remedy.
Warning signs to watch for
Vomiting, hyperactivity, restlessness, rapid breathing, fast heart rate, increased thirst and appetite, vocalizing; large ingestions: tremors, fever.
When to call the vet
Call your veterinarian or a pet poison hotline with the tablet strength and your cat’s weight if any exposure is possible. Cats with existing heart disease should be evaluated even after small exposures.
Common questions
My cat ate a few of my thyroid pills — is that serious?
Yes, potentially — several human tablets can overstimulate a cat’s heart and nervous system. Call your vet or a pet poison hotline with the strength and number eaten.
My cat has hyperthyroidism — should I use the opposite drug?
Never self-treat — feline hyperthyroidism needs veterinary diagnosis and monitoring. Human thyroid medications can dangerously overshoot a cat’s needs.
More medications hazards for cats
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).