Can Dog Hyperthyroidism Cause Aggressive Behavior

by Linda and a Veterinarian Response to Hyperthyroidism
(Calgary, Canada)

Reader Question on dog behavior change and canine hyperthyroidism

Teddy is a 2 year old Spanish Water Dog. He eats 4 cups of high quality high protein dog food per day with a fish base, high in omega 3 and 6, and gets 2 off leash walks (runs) of about 45 minutes each day. He is lean but not skinny and his hair is dry and looks like it needs a good conditioner.

He has always been a very timid and anxious dog and he is now becoming quite fearful aggressive, snapping and snarling if a new dog comes at him from the front, dead on. He barks aggressively while wagging his tail at people coming in to our home even those he has met many times before.

He used to sleep all night but now he wakes us up to go out to urinate and/or defecate once or twice a night. He occasionally eats other dog feces or even his own. We keep our house cool but he often likes to go out in the snow and roll in it, presumably to cool down.

I just feel like something is changing in him, gradually, but ramping up. I wondered if it could be hyperthyroidism.

Vet Suggestion regarding suspected case of canine hyperthyroidism


Hyperthyroidism is extremely rare in dogs, and when it is present it is usually caused by a cancerous thyroid tumor, which is unlikely in a two year old dog.

My biggest concern from your description is your dog’s aggressive behavior. Everything else you mention is either a minor issue (e.g., dry coat) or sounds like typical dog behavior (e.g., rolling in the snow, eating feces, etc.). Some behavioral problems tend to develop when a dog reaches maturity, which your dog just has.

I think your best bet is to make an appointment with a veterinarian who specializes in behavior. He or she can perform a physical exam and general health work up to rule out an underlying disease, and then, assuming your dog is healthy, come up with a behavioral modification plan.

Good luck,

Jennifer Coates, DVM

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