Medical Causes of Aggression in Dogs
The length of this list surprises most owners

Medical causes of aggression in dogs are often overlooked, but it is one of the largest causes for this troubling and very dangerous condition.

Although it can be behavioral, dominance related, or territorial, owners should always look at potential medical causes first, especially if your dog has no previous history of aggression.


Aggression in dogs is technically defined as any type of a behavior that is threatening by a dog and is much broader than just biting.

It also includes snapping, nipping, and lunging, as well as snarling or growling.

It your dog has a long history of aggression, it is most likely a behavioral problem.

But with the vast majority of dogs that suddenly show aggression, you owe it to your loyal friend to understand the medical causes of aggression in dogs.


Medical causes of aggression in dogs start with a condition that can easily affect all dogs, and is a condition that will surprise most owners as the number one underlying potential cause.

Hypothyroidism is most often associated with weight gain, lethargy, and even hair loss in your dog.

However, it is recently become widely accepted that it may also be one of leading, if the leading, cause of dysfunction in your dog.

This condition can cause your dog to gradually or suddenly change their personality.

However, it can be extremely difficult to detect unless an owner is aware of the potential damage it can do.

Hypothyroidism where your dog’s thyroid hormone levels are below the normal ranges is quite easy to detect.

But to complicate things, there is now another range that is being explored by the medical community.

It is technically called a sub-threshold range where your dog is somewhere between normal and the low levels that would provide full operational functions of this critical hormone.

It is this sub-threshold that may be the actual cause of dysfunction and as a result causing your dog to either gradually or very suddenly become aggressive.

With this form of medical causes of aggression in dogs they may show only a couple of the symptoms of a low thyroid.

However, even a couple of the symptoms will tell you that something is wrong, especially if your dog has never shown any type of aggression.

There are numerous symptoms of hypothyroidism, and they include the following: Lethargy, depression, tiring easily, and a sudden weight gain.

Husky eyesIf your dog suddenly truns ugly something is terribly wrong

However, they also include a very sudden intolerance to cold, constipation, diarrhea, and any type of chronic skin condition, especially excessive shedding and hair loss.

With this sub-threshold, your dog may only show one or two of these symptoms but if it is combined with aggression, take your dog to your veterinarian and have them tested as soon as you can.

In most all cases, they will test in the between stage but only have about 25 percent of what would be considered normal.

A dog that has normal thyroid levels usually tests somewhere between 50 percent and 100 percent normal levels.

Neurological Conditions

The next set of medical causes of aggression in dogs is usually the result of neurological problems that can be either congenital or acquired.

If it is congenital, it means your dog was born with the condition, while acquired means they have had a result illness or perhaps an injury.

The most common of these causes is what is referred to as behavioral seizures.

Behavioral seizures do not imply that your dog’s behavior caused the seizure, but rather the seizure causes the behavior.

These forms of seizures are a partial seizure that occurs in the part of your dog’s brain that controls aggression.

There are certain breeds that for reasons yet determined, are much more prone for these seizures.

They include Golden and Chesapeake Bay receivers, Bull Terriers, Poodles, and both Springer as well as Cocker spaniels.

Partial seizures will show you some very distinctive symptoms to watch for.

They include a mood change right before the seizure hits, or very violent and sudden aggressive signs for no real parent reason.

However, your dog may also start to salivate excessively, lose control of their bladder suddenly, or their eyes may become dilated very rapidly.

But the most common sign with this form of medical causes of aggression in dogs will be aggressive posturing.

This aggressive posturing will be very noticeable in a normally mild dog and can be very confusing to the untrained owner as it may last just a few minutes or may last for as long as several days.

But it is definitely a sign that should be taken very seriously as it is not normal at all.

The next form of medical causes of aggression in dogs is referred to as Hydrocephalus, and is a congenital condition.

It is a situation where your dog’s fluid filled spaces in their brain become enlarged and as a result the surrounding brain tissue becomes either very thin or actually compresses.


It is also referred to as water on the brain. Even minor cases can cause slight aggression, but severe cases can cause sudden and violent aggression in your dog.

But the list of medical causes of aggression in dogs does not end there. It can also include Encephalitis, head trauma, brain tumors, as well as epilepsy.

Encephalitis causes inflammation in your dog’s brain, and the result is aggression, as your dog is in a lot of pain and may have no other way of showing it other than the aggression.

Any type of a head trauma can cause regional swelling as well as bleeding that will affect your dogs normal functions.

Brain tumors must also be considered if your dog suddenly becomes aggressive.


Medical causes of aggression in dogs should be the first consideration if a very friendly and mild manner dog suddenly shows aggression in any form.

Your dog may be an extreme barker and be quite defensive in regards to other animals, but in most cases they would never be aggressive unless it is a pure act of defending their owner.

If they do, it is time to have them checked as something may be wrong with them medically.

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