Aggression in dogs can have a medical base. Medically-induced aggression can result in growling, snapping, showing teeth and positioning the body in a dominant stance. Medical causes of aggression in dogs include hypothyroidism, hypoglycemia and brain inflammation.

Brain inflammation that occurs in conditions such as encephalitis is one of the most serious medical causes of aggression in dogs. Rabies and distemper are both types of viral encephalitis. In cases of brain inflammation due to diseases such as encephalitis, the agression is usually a neurological reaction to the affect of the inflammation on the brain.

Hypoglycemia, or low blood sugar, is one of the medical causes of aggression in dogs and may or may not be related to diabetes. Dogs with low blood sugar may feel disoriented and strange. If they feel vulnerable, they may behave aggressively in order to avoid appearing weak. Dogs with low blood sugar may appear glassy-eyed with a staring look. Weakness or collapse may occur. A veterinarian can perform a simple blood test to diagnose hypoglycemia.

Hypothyroidism is one of the most common medical causes of aggression in dogs. Very low thyroid levels affect mood. Thyroid replacement therapy has worked to correct the medical causes of aggression in dogs with hypothyroidism in some cases. Weight gain, lethargy and the loss of a large amount of hair are other symptoms of hypothyroidism.

Hypothyroidism is one of the most studied medical causes of aggression in dogs. Over 50 different breeds and cross breeds are susceptible to hypothyroidism. Behavioral changes, especially aggression, are often the earliest signs of hypothyroidism. Even a dog that was once friendly to strangers may become aggressive if the dog has extremely low thyroid levels. A veterinarian can conduct a blood test to diagnose hypothyroidism.

Medical causes of aggression in dogs may sometimes be more subtle than growling and snapping. A dog may suddenly begin to stand on people's feet or block doorways as a way of exerting dominance. Cognitive Dysfunction Syndrome (CDS), sometimes called "dog's Alzheimer's disease," is thought to be one of the medical causes of aggression in dogs. CDS may lead to aggression due to the loss of brain cells caused by brain degeneration.