Although the classic behavioral changes associated with hypothyroidism in dogs are apathy, lethargy, and cold intolerance, some dogs become aggressive. The four cases presented in this article serve as a reminder that thyroid testing should be included as part of a minimum database during evaluation of aggressive dogs. Three of the dogs showed dominance-related aggression toward family members, and the other showed fear aggression toward strangers. The owner of one dog observed an episode of growling while the dog was alone in a room. All four cases were treated with 20 µg/kg levothyroxine q 12 hr. The owners were also advised to avoid situations that might trigger aggressive behavior.

After 8 months of therapy, the aggression had diminished but not disappeared. One of the dogs, a cocker spaniel, responded poorly to treatment.

Hypothyroidism affects the turnover of serotonin, and low levels of brain serotonin have been linked to dominance aggression in dogs. This could explain why hypothyroidism may reduce the threshold for dominance aggression and other forms of aggression.

Four cases of aggression and hypothyroidism in dogs. Fatjo J, Stub C, Manteca X. Vet Rec 151:547-548, 2002.