While canine hypothyroidism has been diagnosed for many years, references to associated abnormal behaviors typically cite only lethargy. The behaviors mentioned as symptoms have tended to be general and often included within a list of clinical signs that are related to the concomitant lowered metabolic rate or to neurological abnormalities.1–,4 Prior to 1990, there had only been two authors that noted a relationship between aggression and hypothyroidism.5–,7 Since that time, hypothyroid-related aggression has been recognized as a unique type of aggression. Also, a collection of other behavioral signs has been reported in scattered books and journals that have examined this disease and its behavioral relationships more extensively.

Clinical signs of hypothyroidism affect several body systems; therefore, the manifestation of this condition is extremely variable. These various clinical signs may be seen alone, as is often...

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