CANINE AUTOIMMUNE THYROID DISEASE:
COMMON PROBLEM OF PUREBRED DOGS
by W. Jean Dodds DVM
The information provided here outlines an approach that has been
used successfully by the author to reduce the prevalence of
clinically expressed canine thyroid disease within susceptible
families or breeds.
EARLY THYROID DISEASE (THYROIDITIS)
COMPENSATORY AND CLINICAL CANINE HYPOTHYROIDISM
Most of the confusion about the diagnosis and treatment of
thyroid disease in purebred or mixed breed dogs today stems from the
expectation that affected animals must show clinical signs of
inadequate thyroid hormonal production (i.e. hypothyroidism) in
order to have the disease. The term hypothyroidism has been loosely
applied to describe all stages of this disease process whereas
strictly speaking it should be reserved for the end-stages when the
animal's thyroid gland is no longer capable of producing sufficient
hormone(s) to sustain clinical health. At this point, the dog can
express any number of the non-specific multisystem signs of thyroid
dysfunction. But let's start at the beginning.
The most common cause of canine thyroid disease is autoimmune
thyroiditis (estimated 90% of cases). Thyroiditis is an
immune-mediated process that develops in genetically susceptible
individuals and is characterized by the presence of antithyroid
antibodies in the blood or tissues. Thyroiditis is believed to start
in most cases around puberty, and gradually progress through
mid-life and old age to become clinically expressed hypothyroidism
once thyroid glandular reserve has been depleted. During this
process, the animal or person becomes more susceptible to
immune-mediated or other diseases affecting various target tissues
and organs. The prerequisite genetic basis for susceptibility to
this disorder has been in established in humans, dogs and several
other species.
The above explanation helps us to appreciate existing confusion
and controversy within the veterinary profession regarding whether
or not testing or treatment is indicated for dogs that fail to show
typical signs of hypothyroidism. In fact, we have only recently
begun to recognize the subtle signs of early thyroid dysfunction in
dogs as prevalence of the autoimmune form of the condition has
increased within and among dog breeds. Today, some 50 breeds are
genetically predisposed to develop thyroid disease.
GENETIC SCREENING FOR THYROID DISEASE
These thyroid panels and antibody tests can also be used for
genetic screening of apparently healthy animals to evaluate their
fitness for breeding. A bitch with antithyroid antibodies in her
blood may pass these along to her puppies in her colostral milk.
Also, any dog having circulating antithyroid antibodies can
eventually develop clinical symptoms of thyroid or other autoimmune
diseases. Therefore, thyroid screening can be very important for
potential breeding stock.
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 anestrus period for
females following their maiden heat). Anestrus is a time when the
female sexual cycle is quiescent, thereby removing any influence of
sex hormones on baseline thyroid function. 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 is more reliable when they are tested in
anestrus. Testing for health screening is performed at 12-16 weeks
from the onset of the previous heat. In fact, genetic screening of
intact females for other parameters like von Willebrand's disease or
wellness health and reproductive checkups should also be scheduled
in anestrus females. Once the initial thyroid profile is obtained,
dogs and bitches should be rechecked on an annual basis to assess
their own health. Annual results permit comparisons that should
reveal early evidence of developing thyroid disease or dysfunction.
This also allows for early treatment where indicated to abort the
development or advancement of clinical signs associated with
hypothyroidism.
Healthy young dogs (less than 15-18 months of age) should have
thyroid baseline levels for all parameters in the upper 1/2 to 1/3
of the adult normal ranges. In fact, for optimum thyroid function in
screening breeding stock, levels should be at least at the midpoint
of the laboratory normal ranges, because lower levels may well be
indicative of the early stages of thyroiditis among relatives of dog
families known to have thyroid disease.
TREATMENT OF THYROID DISEASE
The new information summarized here has changed our approach to
treatment and control of thyroid disease. In addition to providing
thyroid supplementation for dogs showing the typical signs of
thyroid disease, we now know that treatment of dogs showing the
early stages of thyroiditis (based on the testing described above)
is necessary and important to correct the underlying thyroid
imbalance, reduce the risk of developing other related
immune-mediated disorders, and to control or prevent the process of
thyroiditis from progressing to depletion and exhaustion of the
thyroid gland.
1. Type of Treatment
The treatment of choice because of its wide safety margin and
efficacy is T4 hormone (L - or levothyroxine). The most commonly
used brand names are Soloxine (Daniels) and Synthroid (Flint) and we
recommend either of these over generics especially for the smaller
breeds. Use of T3 hormone (triiodothyronine) is not recommended for
initial use because toxicity can more easily develop with this
product; T3 is the intercellular hormone whereas most of T4 must be
first converted to T3 before it achieves its metabolic effect. In
some cases where the animal's body cannot properly convert T4 to T3,
the dog will need both T4 and T3 therapy to correct the problem. For
this purpose, the general rule of thumb is to give from 2/3 to a
full dose of T4 and a 1/3 dosage of T3 (i.e., 0.1 mg per 10-20
pounds of T4 plus 1 ugm per pound of T3 twice daily). However, no
dog should be treated with these thyroid hormonal preparations
without having proper veterinary testing, medical examination and
follow-up.
2. Frequency of Treatment
Thyroid hormones should always be given twice daily to effect the
best response. Until recently, veterinarians have been advised to
give treatment to effect either once or twice daily because data on
this point was unclear. We now know that the half-life of T4 in the
dog is about 10-12 hours (much shorter than humans); for T3, it's
only 6-8 hours. Thus, about half of the hormone is metabolized and
excreted from the body within 12 hours. Furthermore, twice daily
dosing aids in controlling thyroiditis because it shuts off
pituitary production of TSH by negative feedback in concert with the
half-life of the hormone. In other words, the dog's own thyroid
follicular cells become quiescent and are less likely to stimulate
production of the antithyroid antibodies responsible for the
disease. (Obviously these are simplistic explanations of the complex
metabolic, immunologic and biochemical events involved.) Contrary
to some popular wisdom, treatment with thyroid hormone does not
destroy or suppress the potential of the gland to respond on its own
once treatment is stopped for whatever reason. The latest veterinary
research shows that it takes the thyroid gland up to 30 days to
recover its full potential once therapy is withdrawn. Therefore if
an animal has been medicated, where the diagnosis is unclear,
treatment should be withdrawn (if it's clinically safe to do so) for
30 days before the animal is retested with the complete type thyroid
profile described above.
Follow-up testing after initiating treatment is usually performed
after four to eight weeks of therapy. The sample should be taken 4-6
hours after the morning dosage and optimum results will show thyroid
values in the upper third of normal ranges at the peak time of
absorption. Dosage can then be adjusted accordingly if needed. Dogs
on long term therapy with thyroid hormones should be monitored with
complete panels (not just T4 as you need to be sure the dog's body
is converting the T4 medication properly to T3) on a regular basis
(every 6-12 months).
CLINICAL SIGNS OF CANINE HYPOTHYROIDISM
Alterations in Cellular Metabolism
weakness / stiffness / laryngeal paralysis / facial paralysis /
tragic expression / knuckling or dragging feet / muscle wasting /
megaesophagus / head tilt / drooping eyelids
Neuromuscular Problems
seizures / mental dullness / exercise intolerance / neurologic
signs polyneuropathy / lethargy / weight gain / cold intolerance /
mood swings hyperexcitability / stunted growth / chronic infections
Dermatologic Diseases
dry, scaly skin and dandruff / coarse, dull coat / bilateral
symmetrical hair loss / rat tail, puppy coat / hyperpigmentation /
seborrhea or greasy skin pyoderma or skin infections / myxedema /
chronic offensive skin odor
Reproductive Disorders
infertility of either sex / lack of libido / testicular atrophy /
hypospermia aspermia / prolonged interestrus interval / absence of
heat cycles / silent heats / pseudopregnancy / weak, dying or
stillborn pups
Cardiac Abnormalities
slow heart rate (bradycardia) / cardiac arrhythmias /
cardiomyopathys
Gastrointestinal Disorders
constipation / diarrhea / vomiting
Hematological Disorders
bleeding / bone marrow failure / low red blood cells / low white
blood cells / low platelets
Ocular Diseases
corneal lipid deposits / corneal ulceration / uveitis
Keratococonjunctivitis / sicca or dry eye / infections of eyelid
glands (Meibomian gland)
Other Associated Disorders
lgA deficiency / loss of smell (dysosmia) / loss of taste /
glycosuria / chronic active hepatitis / other endocrinopathies
adrenal, pancreatic, parathyroid
Related Articles on Thyroid Disease
BEHAVIORAL CHANGES ASSOCIATED WITH THYROID DYSFUNCTION IN DOGS
THYROID CAN ALTER BEHAVIOR
CANINE THYROID DISEASE
