medical causes of aggression

Medical Causes of Aggression In Dogs

Medical Causes of Canine Aggression

Aggression in dogs is defined as a threat of harmful behavior directed at another animal or person. It may involve snarling, growling, snapping, nipping, biting, or lunging. A dog may act aggressively for either behavioral or medical reasons, or a combination of both. Here are some of the medical conditions that may contribute to or cause canine aggression.

Hypothyroidism in Dogs

All veterinarians are aware of hypothyroidism, a condition in which the thyroid gland produces less thyroid hormone than normal. Hypothyroidism can be suspected from overt signs, such as increased body weight, lethargy, hair loss, etc. Blood levels of thyroid hormone can confirm the diagnosis. Recently, however, it has come to light that hypothyroidism is not simply an all-or-none condition; there can be various degrees of dysfunction.

Somewhere between “normal” and hypothyroid are dogs whose thyroid hormone levels are lower than necessary for optimal function but whose levels are still technically within the normal range. In this situation, only one or two of the clinical signs of hypothyroidism may be present, and even so, their extent may only be subtle. This situation can be referred to as “sub-clinical” or “sub-threshold” hypothyroidism (i.e. below the threshold for a definitive diagnosis).

For example, a 2-year-old golden retriever that is shedding excessively and showing aggression may have thyroid hormone levels in the 25th percentile in the normal range. A healthy, active dog of this age should have her thyroid hormone levels between the 50th to 100th percentile of the normal range for optimal well being. If thyroid hormone levels are elevated to the optimum end of the range by giving synthetic thyroid hormone, dramatic improvements in the dog’s physical status, mood, and behavior can result.

Sub-clinical hypothyroidism is diagnosed with a number of other factors in mind such as:

Congenital or Acquired Neurological Problems in Dogs

If a dog is born with a neurological problem or develops one (through injury or illness), its perception and judgment may be affected, causing inappropriate behavior. The following are some of the problems that can lead to aggressiveness:


Encephalitis (Bacterial or Viral)

Head Trauma

Brain Tumors


Behavioral Seizures

Partial seizures occurring in a region of the brain that controls aggression (e.g. hypothalamus or limbic system) can result in sudden unprovoked aggression. Certain breeds of dogs are known for this sudden, haphazard, and sometimes violent form of aggression. Included are: Springer spaniels, cocker spaniels, Chesapeake Bay retrievers, bull terriers, poodles, and golden retrievers. The clinical signs of seizure-related aggression are completely different from any of the other types of aggression mentioned above. They are as follows: