Can Hypothyroidism Cause Dogs to Be Aggressive?

Golden retrievers are most commonly diagnosed with hypothyroidism.
i Janie Airey/Lifesize/Getty Images

If your normally friendly dog has developed a Jekyll and Hyde personality, it's time to have the dog's thyroid levels checked by a veterinarian. Once a dog is diagnosed with hypothyroidism, treatment can begin and most dogs return to their peaceful ways within a week.


Hypothyroidism is caused by low levels of thyroid hormones caused by inflammation or shrinking of the thyroid gland. Some breeds are genetically predisposed to hypothyroidism, most commonly golden retrievers, doberman pinschers and Irish setters. All dog breeds can be diagnosed with hypothyroidism, but the diagnosis occurs less frequently in small and giant breeds. The most common cause of hypothyroidism is autoimmune thyroiditis where the dog's immune system attacks the thyroid cells.


Aggression is not a common symptom of hypothyroidism, but some dogs do display significant personality changes. Hypothyroidism is mostly known for causing excessive hair loss, a dull coat, mental dullness, obesity, intolerance to physical activity and cold temperatures, as well as skin changes. A dog's skin can thicken and display color changes. The skin changes make dogs prone to skin infections. In some cases, your dog is so weak from the low thyroid levels that he'll start walking abnormally. If your dog is displaying any of these signs in combination with aggression, hypothyroidism is a possibility.


It's unclear what aspect of hypothyroidism causes aggression, especially since sudden aggression with low thyroid levels is rare. One theory is the reduced thyroid levels has an impact on the neuroendocrine system that controls the dog's stress response. The stress hormone, cortisol, is elevated with some dogs diagnosed with hypothyroidism. Chronically elevated levels of cortisol are linked to depression and abnormal mental function, so these factors could play a role in the dog's aggressive response. Sometimes the aggression displayed is subtle, but it can escalate to dangerous levels if left untreated.

Borderline Hypothyroidism

Some studies suggest that dogs with thyroid levels that appear on the low side or just slightly below normal are prone to anxiety and aggression. Not all veterinarians support this theory, but if your vet is on board, a four- to six-week trial treatment for hypothyroidism is often efficient to see if improvement occurs. If your veterinarian is against borderline hypothyroidism being the cause of sudden aggression, seeking a second opinion with another veterinarian wouldn't hurt anything.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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