Introduction

Hypothyroidism is the most common endocrine disorder of canines, and up to 80% of cases
result from autoimmune (lymphocytic) thyroiditis. The heritable nature of this disorder poses
significant genetic implications for breeding stock. Thus, accurate diagnosis of the early
compensatory stages of canine autoimmune thyroiditis leading up to hypothyroidism affords
important genetic and clinical options for prompt intervention and case management.
Although thyroid dysfunction is the most frequently recognized endocrine disorder of pet
animals, it is often difficult to make a definitive diagnosis. As the thyroid gland regulates
metabolism of all body cellular functions, reduced thyroid function can produce a wide range of
clinical manifestations. Many of these clinical signs mimic those resulting from other causes and
so recognition of the condition and interpretation of thyroid function tests can be problematic
(Table 1).

Baseline Thyroid Profiles
A complete baseline thyroid profile is measured and typically includes total T4, total T3, free T4,
free T3, T3AA and T4AA, and can include cTSH and/or TgAA. The TgAA assay is especially
important in screening breeding stock for heritable autoimmune thyroid disease.
The normal reference ranges for thyroid analytes of healthy adult animals tend to be similar for
most breeds of companion animals. Exceptions are the sighthound and giant breeds of dogs
which have lower basal levels. Typical thyroid levels for healthy sighthounds, such as retired
racing greyhounds, are at or just below the established laboratory reference ranges, whereas
healthy giant breeds have optimal levels around the midpoint of these ranges.
Similarly, because young animals are still growing and adolescents are maturing, optimal thyroid
levels are expected to be in the upper half of the references ranges. For geriatric animals, basal
metabolism is usually slowing down, and so optimal thyroid levels are likely to be closer to
midrange or even slightly lower.

Genetic Screening for Thyroid Disease
Most cases of thyroiditis have elevated serum TgAA levels, whereas only about 20-40% of cases
have elevated circulating T3 and/or T4 AA. Thus, the presence of elevated T3 and/or T4 AA
confirms a diagnosis of autoimmune thyroiditis but underestimates its prevalence, as negative
(non-elevated) autoantibody levels do not rule out thyroiditis. Measuring TgAA levels also
permits early recognition of the disorder, and facilitates genetic counselling. Affected dogs
should not be used for breeding (Table 2). 2

The commercial TgAA test can give false negative results if the dog has received thyroid
supplement within the previous 90 days, thereby allowing unscrupulous owners to test dogs
while on treatment to assert there normalcy, or to obtain certification with health registries such
as the OFA Thyroid Registry. False negative TgAA results also can occur in about 8% of dogs
verified to have high T3AA and/or T4AA. Furthermore, false positive TgAA results may be
obtained if the dog has been vaccinated within the previous 30-45 days, or in some cases of nonthyroidal
illness. Vaccination of pet and research dogs with polyvalent vaccines containing
rabies virus or rabies vaccine alone was recently shown to induce production of
antithyroglobulin autoantibodies, a provocative and important finding with implications for the
subsequent development of hypothyroidism.

A population study of 287,948 dogs was recently published by the MSU Animal Health
Diagnostic Laboratory. Circulating thyroid hormone autoantibodies (T3AA and/or T4AA)) were
found in 18,135 of these dogs (6.3%). The 10 breeds with the highest prevalence of thyroid AA
from their study were: Pointer, English Setter, English Pointer, Skye Terrier, German Wirehaired
Pointer, Old English Sheepdog, Boxer, Maltese, Kuvasz, and Petit Basset Griffon Vendeen.
Prevalence was associated with body weight and was highest in dogs 2-4 years old. Females
were significantly more likely to have thyroid AA than males.

A bitch with circulating thyroid AA has the potential to pass these along to the puppies
transplacentally as well as via the colostrum. Furthermore, any dog having thyroid AA may
eventually develop clinical symptoms of thyroid disease and/or be susceptible to other
autoimmune diseases. Thyroid screening is thus very important for selecting potential breeding
stock as well as for clinical diagnosis.

Thyroid testing for genetic screening purposes is less likely to be meaningful before puberty.
Screening is initiated, therefore, once healthy dogs and bitches have reached sexual maturity
(between 10-14 months in males and during the first anestrous period for females following their
maiden heat). As the female sexual cycle is quiescent during anestrus, any influence of sex
hormones on baseline thyroid function will be minimized. This period generally begins 12
weeks from the onset of the previous heat and lasts one month or longer. The interpretation of
results from baseline thyroid profiles in intact females will be more reliable when they are tested
in anestrus. In fact, genetic screening of intact females for other disorders such as von
Willebrand disease (vWD), hip dysplasia, and wellness or reproductive checkups (vaginal
cultures, hormone testing) is best scheduled during anestrus. Once the initial thyroid profile is
obtained, dogs and bitches should be rechecked on an annual basis to assess their thyroid
function and overall health. Generation of annual test results provides comparisons that permit
early recognition of developing thyroid dysfunction. This allows for early treatment, where
indicated, to avoid the appearance or advancement of clinical signs associated with
hypothyroidism.

Canine autoimuune thyroid disease is very similar to Hashimoto’s thyroiditis of humans, which
has been shown to be associated with human major histocompatibility complex (MHC) tissue
types. A similar association with canine MHC genes in hypothyroid dogs has recently been
reported in Doberman Pinschers, English Setters and Rhodesian Ridgebacks, who share a rare dog
leukocyte antigen (DLA) class II haplotype which contains a unique DLA-DQA1*00101 genetic
determinant. While the presence of this determinant doubles the risk of a dog developing
hypothyroidism, it was not found in boxers affected with thyroiditis, nor was it found in the Shih
Tzu, Yorkshire Terrier, or Siberian Husky, although more studies are needed in these and other
3 susceptible breeds to establish their true status with respect to this marker DLA antigen. This
exciting finding of a common genetic determinant associated with thyroid disease in several
breeds hopefully will lead to development of a genetic marker test to identify affected breeding
stock and allow for selective breeding to reduce disease incidence in pure-bred dogs.

Polyglandular Autoimmunity
Individuals genetically susceptible to autoimmune thyroid disease may also become more
susceptible to immune-mediated diseases affecting other target tissues and organs, especially the
bone marrow, liver, adrenal gland, pancreas, skin, kidney, joints, bowel, and central nervous
system. The resulting “polyglandular autoimmune syndrome” of humans is becoming more
commonly recognized in the dog, and probably occurs in other species as well. The syndrome
tends to run in families and is believed to have an inherited basis. Multiple endocrine glands and
nonendocrine systems become involved in a systemic immune-mediated process. This multiple
endocrinopathy often occurs in patients with underlying autoimmune thyroid disease (hypo- or
hyperthyroidism) and concurrent Addison’s disease, diabetes, reproductive gonadal failure, skin
disease and alopecia, and malabsorption syndrome. The most common nonendocrinologic
autoimmune disorders associated with this syndrome are autoimmune hemolytic anemia (AIHA),
idiopathic thrombocytopenic purpura (ITP), chronic active hepatitis, and immune-complex
glomerulonephritis (systemic lupus erythematosus; SLE).

The most commonly recognized polyglandular endocrinopathy of dogs is Schmidt’s syndrome
(thyroiditis and Addison’s disease). Examples of breeds genetically predisposed to this disorder
include the Standard Poodle, Old English Sheepdog, Bearded Collie, Portuguese Water Dog,
Nova Scotia Duck Tolling Retriever, and Leonberger, although any breed or mixed breed can be
affected. Our study cohort of 162 cases of autoimmune blood and endocrine disorders in Old
English Sheepdogs (1980-1989) included 115 AIHA and/or ITP, 99 thyroid disease, 23
Addison’s disease, 7 vaccine reactions, 3 SLE, 2 diabetes, 1 rheumatoid arthritis and 1
hypoparathyroidism. The group comprised 110 females (15 spayed) and 52 males (3 neutered).
Seven of the most recent 103 cases had two or more endocrine disorders, and 101 of the 108
cases where pedigrees were available showed a familial relationship going back several
generations. Data from surveying the Bearded Collie breed reported 55 hypothyroid, 17
Addison’s disease, and 31 polyglandular autoimmunity (5 were hypothyroid).

Aberrant Behavior and Thyroid Dysfunction
The principal reason for pet euthanasia stems not from disease, but undesirable behavior. While
this abnormal behavior can have a variety of medical causes, it also can reflect underlying
problems of a psychological nature.

An association between behavioral and psychologic changes and thyroid dysfunction has been
recognized in humans since the 19th century. In a recent study, 66% of people with attention
deficit-hyperactivity disorder were found to be hypothyroid, and supplementing their thyroid
levels was largely curative. Furthermore, an association has recently been established between
aberrant behavior and thyroid dysfunction in the dog, and has been noticed in cats with
hyperthyroidism. Typical clinical signs include unprovoked aggression towards other animals
and/or people, sudden onset of seizure disorder in adulthood, disorientation, moodiness, erratic
temperament, periods of hyperactivity, hypoattentiveness, depression, fearfulness and phobias,
anxiety, submissiveness, passivity, compulsiveness, and irritability. After episodes, most of the
animals appeared to come out of a trance like state, and were unaware of their bizarre behavior.
4

The mechanism whereby diminished thyroid function affects behavior is unclear. Hypothyroid
patients have reduced cortisol clearance, as well as suppressed TSH output and lowered
production of thyroid hormones. Constantly elevated levels of circulating cortisol mimic the
condition of an animal in a constant state of stress. In people and seemingly in dogs, mental
function is impaired and the animal is likely to respond to stress in a stereotypical rather than
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 to ambient neurotransmitter levels.

Investigators in recent years have noted the sudden onset of behavioral changes in dogs around
the time of puberty or as young adults. Most of the dogs have been purebreds or crossbreeds,
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 and ticks, 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.

Many of these dogs belong to a certain group of breeds or dog families susceptible to a variety of
immune problems and allergies (e.g. Golden Retriever, Akita, Rottweiler, Doberman Pinscher,
English Springer Spaniel, Shetland Sheepdog, and German Shepherd Dog). The clinical signs in
these animals, before they show the sudden onset of behavioral aggression, can include minor
problems such as inattentiveness, fearfulness, seasonal allergies, skin and coat disorders, and
intense itching. These may be early subtle signs of thyroid dysfunction, with no other typical
signs of thyroid disease being manifested.

The typical history starts out with a quite, well-mannered and sweet-natured puppy or young
adult dog. The animal was outgoing, attended training classes for obedience, working, or dog
show events, and came from a reputable breeder whose kennel has had no prior history of
producing animals with behavioral problems. At the onset of puberty or thereafter, however,
sudden changes in personality are observed. Typical signs can be incessant whining,
nervousness, schizoid behavior, fear in the presence of strangers, hyperventilating and undue
sweating, disorientation, and failure to be attentive. These changes can progress to sudden
unprovoked aggressiveness in unfamiliar situations with other animals, people and especially
with children.

Another group of dogs show seizure or seizure-like disorders of sudden onset that can occur at
any time from puberty to mid-life. These dogs appear perfectly healthy outwardly, have normal
hair coats and energy, but suddenly seizure for no apparent reason. The seizures are often spaced
several weeks to months apart, may coincide with the full moon, and can appear in brief clusters.
In some cases the animals become aggressive and attack those around them shortly before or
after having one of the seizures. Two recent cases involved young dogs referred for sudden onset
seizure disorder shortly after puberty. Both dogs were found to have early onset autoimmune
thyroiditis, which was clinically responsive to thyroid supplementation, to the extent that
anticonvulsant medications could be gradually withdrawn. The numbers of animals showing
these various types of aberrant behavior appear to be increasing in frequency over the last
decade. 5

In dogs with aberrant aggression, a large collaborative study between our group and Dr. Dodman
and colleagues at Tufts University School of Veterinary Medicine 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.

Tables 3-4 summarize results of complete thyroid diagnostic profiling on 634 canine cases of
aberrant behavior, compiled by this author in collaboration with Drs. Nicholas Dodman, Linda
Aronson, and Jean DeNapoli of Tufts University School of Veterinary Medicine, North Grafton,
MA. Ninety percent (568 dogs) were purebreds and 10% were mixed breeds. There was no sex
predilection found in this case cohort, whether or not the animals were intact or neutered. Sixtythree
percent of the dogs had thyroid dysfunction as judged by finding 3 or more abnormal
results on the comprehensive thyroid profile. The major categories of aberrant behavior were
aggression (40% of cases), seizures (30%), fearfulness (9%), and hyperactivity (7%); some dogs
exhibited more than one of these behaviors (Table 4). Within these 4 categories, thyroid
dysfunction was found in 62% of the aggressive dogs, 77% of seizuring dogs, 47% of fearful
dogs, and 31% of hyperactive dogs.

Outcomes of treatment intervention with standard twice daily doses of thyroid replacement were
evaluated in 95 cases, and showed a significant behavioral improvement in 61% of the dogs. 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
behavior or other habit modification over the same time period, only 11 dogs improved more
than 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.

Our ongoing study now includes over 1500 cases of dogs presented to veterinary clinics for
aberrant behavior. The first 499 cases have been analyzed independently by a neural network
correlative statistical program. Results showed a significant relationship between thyroid
dysfunction and seizure disorder, and thyroid dysfunction and dog-to-human aggression.
Collectively, these findings confirm the importance of including a complete thyroid antibody
profile as part of the laboratory and clinical work up of any behavioral case.

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Table 1. CLINICAL SIGNS OF CANINE HYPOTHYROIDISM

Alterations in Cellular Metabolism
lethargy weight gain
mental dullness cold intolerance
exercise intolerance mood swings
neurologic signs hyperexcitability
polyneuropathy stunted growth
seizures chronic infections
Neuromuscular Problems
weakness knuckling or dragging feet 8
stiffness muscle wasting
laryngeal paralysis megaesophagus
facial paralysis head tilt
"tragic" expression drooping eyelids
incontinence ruptured cruciate ligament
Dermatologic Diseases
dry, scaly skin and dandruff chronic offensive skin odor
coarse, dull coat bilaterally symmetrical hair loss
"rat tail"; "puppy coat" seborrhea with greasy skin
hyperpigmentation seborrhea with dry skin
pyoderma or skin infections myxedema
Reproductive Disorders
infertility prolonged interestrus interval
lack of libido absence of heat cycles
testicular atrophy silent heats
hypospermia pseudopregnancy
aspermia weak, dying or stillborn pups
Cardiac Abnormalities
slow heart rate (bradycardia)
cardiac arrhythmias
cardiomyopathy
Gastrointestinal Disorders
constipation
diarrhea
vomiting
Hematologic Disorders
bleeding
bone marrow failure
low red blood cells (anemia), white blood cells, platelets
Ocular Diseases
corneal lipid deposits corneal ulceration
uveitis keratoconjunctivitis sicca or "dry eye"
infections of eyelid glands Vogt-Koyanagi-Harada syndrome
(Meibomian gland)
Other Associated Disorders
IgA deficiency loss of smell (dysosmia)
loss of taste glycosuria
other endocrinopathies chronic active hepatitis
adrenal
pancreatic
parathyroid 9

Table 2. DIAGNOSIS OF THYROID DISEASE
Χ Complete Basic Profile
-- (T4, T3, FT4, FT3, T4AA, T3AA)
Χ Additional Tests
-- (TSH, TgAA)
Χ Older Tests (T4, T4 + T3)
Serum T4 and/or T3 alone are not reliable for diagnosis because:
-- overdiagnose hypothyroidism
-- underdiagnose hyperthyroidism
-- fail to dectect early compensatory disease and thyroiditis
-- influenced by nonthyroidal illness and certain drugs
Χ Newer Tests
Free (Unbound) T4
Less likely to be influenced by nonthyroidal illness or drugs
Valid
-- equilibrium dialysis
-- solid-phase analog RIA
-- chemiluminescence solid-phase
Less reliable -- liquid-phase analog RIA
Endogenous Canine TSH
In primary hypothyroidism, as free T4 levels fall, pituitary output of TSH rises
-- elevated TSH usually indicates primary thyroid disease
-- 20-40% discordancy observed between expected and actual findings
-- published normal ranges may need revising upwards
-- affected by concomitant chronic renal disease
Canine TgAA
Thyroglobulin autoantibodies are present in serum of
cases with lymphocytic thyroiditis.
-- positive results confirm diagnosis ; 8% false negative
-- 20-40% of cases have circulating T3 and/or T4AA
-- allows for early diagnosis and genetic counseling 10

Table 3. CANINE ABERRANT BEHAVIOR *
___________________________________________________________________
Total No. Purebreds Mixed Thyroid Euthyroid
Cases Breeds Dysfunction
____________________________________________________________________
634 568 66 401 † 233
____________________________________________________________________
* Mean Age, 3.7 years ( Range 0.5-12 years ). Median Age, 2.5 years.
Table 4. MOST COMMONLY REPRESENTED BREEDS WITH THYROID
DYSFUNCTION AND ABERRANT BEHAVIOR *
_________________________________________________________________________
Breed† Thyroid Aggression Seizures Fearful Hyperactive
Dysfunction
401/634 251/634 189/634 55/634 42/634
(63%) (40%) (30%) (9%) (7%)_____
Golden
Retriever 50/73 12/16 22/30 4/6 1/6
German
Shepherd 34/53 10/22 14/16 3/7 2/2
Akita 27/38 24/33 0/1 0 0/2
Labrador
Retriever 8/30 6/11 12/16 2/15 0/3
Shetland
Sheepdog 14/25 3/6 2/3 2/4 3/3
Collie 8/9 0 7/7 0 0
English
Setter 4/6 1/1 0 1/3 1/2
Other
Purebreds 217/334 89/135 72/93 10/15 5/16
Mixed
Breed 39/66 11/27 16/23 4/5 1/8
______________________________________________________________________________
Totals 401/634 156/251 145/189 26/55 13/42
(63%) (62%) (77%) (47%) (31%)___________
* Some dogs had more than 1 abnormal behavior. Numerator = Thyroid dysfunction