North America, the principal reason for pet euthanasia stems not from disease,
but undesirable behavior. While this abnormal behavior in dogs and cats
can have a variety of medical causes, it also can reflect underlying problems of
a psychological nature.
has been shown to play an important role in the behavior of both animals and
humans. The role of inheritance in behavior was reviewed by Plomin (Science
248:183-188, 1990), who pointed out that the genetic influence on behavioral
disorders rarely accounts for more than half of the phenotypic expression of
behavioral differences. Each of the multiple genes involved has a small
effect on behavior. Development and application of newer techniques in
molecular biology offers the promise of identifying the DNA marker sequences
responsible for behavioral variation. However, behavior is the most
complex phenotype because it reflects not only the functioning of the whole
organism but also is dynamic and changes in response to environmental
influences. With respect to animal behavior, applied behavioral genetics
was first studied several thousand years ago because animals were bred and
selected for their behavior as much as their conformation. The results can
be attested to by the dramatic differences in behavior and physique among
various dog breeds. Today these breeds have a great range of genetic and
investigators in recent years, have noted the sudden onset of behavioral changes
in dogs around the time of puberty. Most of the dogs have been purebreds
or crossbreds with an apparent predilection for certain breeds. For a
significant proportion of these animals, neutering does not alter the symptoms
and in some cases the behaviors intensify. The seasonal effects of
allergies to inhalants and ectoparasites such as fleas, followed by the onset of
skin and coat disorders including pyoderma, allergic dermatitis, alopecia, and
intense itching, have also been linked to changes in behavior.
interesting association which as been increasing in frequency is the link
between thyroid dysfunction and aberrant behavior. Typical clinical signs
include unprovoked aggression towards other animals and/or people, sudden onset
of a seizure disorder in adulthood, disorientation, moodiness, erratic
temperament, periods of hyperactivity, hypoattentiveness, depression,
fearfulness and phobias, anxiety, submissiveness, passivity, compulsiveness, and
irritability. After the episodes, a majority of the animals were noted to
behave as if they were coming out of a trance- like state and were unaware of
their previous behavior.
similar association between behavioral and psychologic changes and thyroid
dysfunction has been recognized in humans since the 19th century, and more
recently has been noticed in cats with hyperthyroidism. In a recent human
study, 66% of patients with attention deficit-hyperactivity disorder were found
to be hypothyroid, and supplementing their thyroid levels was largely curative.
mechanism whereby diminished thyroid function affects behavior is unclear.
Hypothyroid patients have reduced cortisol clearance, and the constantly
elevated levels or circulating cortisol mimic the condition of an animal in a
constant state of stress, as well as suppressed TSH output and production of
thyroid hormones. In humans and seemingly in dogs, mental function is
impaired and the animal is likely to respond to stress in a stereotypical rather
than a reasoned fashion. Chronic stress in humans has been implicated in
the pathogenesis of affective disorders such as depression. Major
depression has been shown in imaging studies to produce changes in neural
activity or volume in areas of the brain which regulate aggressive and other
behaviors. Dopamine and serotonin receptors have been clearly demonstrated
to be involved in aggressive pathways in the CNS. Hypothyroid rats have
increased turnover of serotonin and dopamine receptors, and an increased
sensitivity ot ambient neurotransmitter levels. In dogs with aberrant
aggression, a large collaborative study at Tufts University has shown a
favorable response to thyroid replacement therapy within the first week of
treatment, whereas it took about three weeks to correct their metabolic
deficit. Dramatic reversal of behavior with resumption of previous
problems has occurred in some cases if only a single dose is missed. A
similar pattern of aggression responsive to thyroid replacement has been
reported in a horse.
1 and 2 summarize results of complete thyroid diagnostic profiling on 634 canine
cases of aberrant behavior, compiled by the authors in collaboration with Drs.
Nicholas Dodman, and Jean DeNapoli of Tufts University School of Veterinary
Medicine, North Grafton, MA.
percent (568 dogs) were purebreds and 10% were mixed breeds.
was no sex predilection found in this case cohort, whether or not the animals
were intact or neutered.
had thyroid dysfunction as judged by finding 3 or more abnormal results on the
comprehensive thyroid profile
major categories of aberrant behavior were: aggression (40% of cases),
seizures (30%), fearfulness (9%), and hyperactivity (7%); some dogs exhibited
more than 1 of these behaviors.
dysfunction was found in 62% of the aggressive dogs, 77% of seizuring dogs, 47%
of fearful dogs, and 31% of hyperactive dogs.
of treatment intervention with standard twice daily doses of thyroid replacement
were evaluated in 95 cases. Of these, 58 dogs had greater than 50%
improvement in their behavior as judged by a predefined 6-point subjective scale
(34 were improved >75%), and another 23 dogs had >25 but <50%
improvement. Only 10 dogs experienced no appreciable change, and 2 dogs
had a worsening of their behavior. When compared to 20 cases of dominance
aggression treated with conventional behavioral or other habit modification over
the same time period, only 11 dogs improved >25% and of the remaining 9
cases, 3 failed to improve and 3 were euthanized or placed in another
home. These initial results are so promising that complete thyroid
diagnostic profiling and treatment with thyroid supplement, where indicated, is
warranted for all cases presenting with aberrant behavior.
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1. Canine Aberrant Behavior *
Mean Age, 3.7 years (Range 0.5-12 years). Median Age, 2.5 years.
2. Most Commonly Represented Breeds with Thyroid Dysfunction and Aberrant
Some dogs had more than 1 abnormal behavior. Numerator = Thyroid
Dysfunction. Denominator - Aberrant behavior