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Central
Rockies Schipperke Club
of
Greater Denver
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| Schipperke
Health |
Genetic
disorder - Thyroid
disease
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CANINE AUTOIMMUNE THYROID DISEASE
AND SYMPTOMS OF HYPOTHYROIDISM
CANINE AUTOIMMUNE THYROID DISEASE: COMMON PROBLEM
OF PUREBRED DOGS
by W. Jean Dodds DVM
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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
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Send correspondence to:
Sue Geiger
Central Rockies Schipperke Club
of Greater Denver
3228 S. Garrison St. Apt.299
Lakewood, Colorado 80227
Email: CRSCSecretary@schipperke.com
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