I don't have any experience with this, but I have read on this site a while back, someone who had the same issues, and it ended up being a low thyroid problem. Maybe worth checking this out, blood tests? Good luck. Keep us posted.
I copied and pasted a couple of articles for you on some possible causes of canine aggression, including hypothyroidism.
What Could Cause Sudden Aggression in a Dog?
By Melinda Weaver, eHow Contributor
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These dogs are showing classic signs of aggression: wide pupils, lunging and growling.
Dog aggression can manifest itself in many ways, such as through growling, snapping, nipping, biting or lunging. A sudden onset of aggression is often caused by a medical condition, although behavioral problems can also play a part.
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Significance
Sudden behavior changes, such as aggression, fear or aloofness, can be scary, but they are often your dog's way of dealing with something that doesn't feel right in his body. To treat the behavior, you must discern the cause.
Pain
If your dog has an injury or has recently developed joint or muscle pain, he may react with sudden aggression if you attempt to touch the painful area. Make your dog as comfortable as possible and avoid touching tender areas while you consult your veterinarian.
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Illness
According to PetPlace.com, hypothyroidism--when the thyroid produces insufficient hormones--and hypoglycemia--or low blood sugar--are common causes of sudden aggression. Illnesses such as distemper and rabies can also result in sudden aggression. Seizures, as with epilepsy, can have the appearance of aggression, when in fact your dog is merely seizing and not lashing out.
Trauma
Head trauma or brain tumors can cause sudden behavioral changes. Emotional trauma can have the same effect. Dogs at home alone can be traumatized and exhibit aggression from an event that happens without your knowledge, such as a thunderstorm or robbery.
Treatment
Visit your veterinarian for a complete health check, including a blood panel, fecal screening, urinalysis and potentially X-rays or CAT scans. If your dog has a clean bill of health, consult a trainer who can help you determine the cause and reduce your dog's fears.
Read more: What Could Cause Sudden Aggression in a Dog? | eHow.com
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Medical Causes of Aggressive Behaviors in Dogs
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It is an unfortunate fact that often dog owners feel compelled to give up on their canine friend as soon as he displays aggressive behaviors by either giving him away, or worse, surrendering him to a shelter or permanently solving the problem by putting him down. Yet, there are cases where aggressive behaviors in dogs can be traced down to unforeseen medical conditions. What owners therefore may perceive as out of the blue, unprovoked aggressive attacks, can be instead attributed to something as justifiable as it can be from a dog's perspective: pain.
Veterinarians know this very well. In every veterinarian office tucked away in a handy drawer lies a set of different sized muzzles ready to be used. They know as a fact that even the most docile dogs may easily turn into vicious and snarling monsters at the sight of a needle or upon being treated for a painful wound. Yet, while in these cases, the pain is visible to the eye and quite obvious, there are sometimes medical conditions in aggressive dogs causing very subtle symptoms that can be hardly recognized and detected by even the most attentive owners.
Owners of normally well mannered and well tempered dogs should suspect a medical condition when their dog acts aggressively out of the blue or in a gradual, yet, steady manner. Medical conditions should be suspected as well in stray dogs or rescue dogs with an unknown medical history. However, not all owners are willing to give these dogs the benefit of doubt, either out of ignorance (not knowing that there are actually conditions causing aggressive behaviors) or out of fear (the dog is a scary, must get rid of him as soon as possible!).
Truth is, an aggressive dog is a scary dog, especially when the aggression is targeted towards children and people. Such dogs are perceived as a liability, an animal too dangerous to be handled and is easily converted from man's best to man's worst enemy. A once much loved dog has progressively transitioned into a snarling vicious creature ready to bite. While many cases of dog aggression are behavioral in nature, it would be unjust to simply assume so without first attempting to rule out something medically going on.
This is why it is imperative to have a dog seen by a veterinarian right before calling a behaviorist or sending the dog to a trainer. Indeed, reputable dog behaviorists should ask for proof of a medical exam before seeing the dog. A dog behaviorist will never be able to fix a dog acting aggressively because of an underlying medical problem! Following are some medical conditions known to cause aggressive behaviors in dogs.
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Medical Causes of Dog Aggression
Chronic Pain
An underlying ear infection may cause a dog to growl or bite upon being touched near the head. Spinal and neck problems may cause aggression when the dog's collar is touched to put on a leash. Chronic disorders such as arthritis and hip dysplasia may cause grumpy behaviors. A recent study revealed that sudden outbreaks of aggression are often pain related.
• Hypothyroidism
This condition affecting the endocrine system of dogs is caused by a low count of thyroid hormones. This condition can be easily detected through a simple blood test. Affected dogs typically develop increased weight gain, hair loss, lethargy, low tolerance of cold, and behavior changes such as anxiety, fear and aggression. It is quite unusual for dog to develop aggression as a stand alone symptom when having this condition, however running a thyroid test is still worthy.The condition is easily treated with the long term administration of thyroid pills which will dramatically improve the dog's condition.
• Seizures
Dogs may develop aggressive behaviors in the post-ictal phase, following a seizure. Partial seizures in dogs that affect a particular portion of the brain responsible for regulating aggression may be also a contributing factor for aggression and aberrant behaviors. These types of seizures are often found in certain dog breeds such as a Cocker Spaniels, Springer Spaniels and Chesapeake Bay Retrievers.
• Brain Conditions
Trauma affecting certain regions of the brain may cause neurological symptoms including aggressive behaviors. These may take place after the dog has sustained brain damage. Brain cancer found often in senior dogs may be another cause for such behavior changes. Encephalitis, the inflammation of the brain may cause significant aggression in affected dogs. Rabies and Distemper are bacterial forms of encephalitis. Hydrocephalus, a congenital condition where the brain's ventricles become enlarged may cause a variety of neurological symptoms, sometimes including aggression
There are many more examples of medical conditions causing aggression in dogs. A dog reacting aggressively upon being pet on the head may be suffering from an undiagnosed ear infection, a dog suffering from arthritis may react defensively when laying down and feeling like he cannot remove himself from an irritating situation such as when being pestered by a child. A hearing impaired dog may easily startle and bite out of fear when approached without notice. A dog may turn grumpy when dealing with tooth pain. These are just examples of dogs that are dealing with pain and medical conditions that often cannot be detected by the most well meaning owners.
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