You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Middle-aged dogs sometimes develop aggression which can be associated with atypical hypothyroidism

An unusual cause of aggression in dogs (estimated to account for about 2% of cases) is hypothyroidism. These dogs may not show any of the other signs typical of hypothyroidism such as hair coat loss or weight gain. Initial signs occur gradually and involve uncharacteristic aggression towards owners when being moved. Affected animals are irascible, grumpy individuals which occasionally "snap".

Circulating thyroid hormone concentrations are low and thyroid replacement therapy successfully resolves the problem. 

Updated October 2013