AUTOIMMUNE THYROID DISEASE:
COMMON PROBLEM OF PUREBRED DOGS
by W. Jean Dodds DVM
As so many breeds of dogs including the Beagle
suffer from familial thyroid disease, a concerted effort needs to be made by dog breeders
and owners to screen breeding stock for evidence of thyroid dysfunction prior to their use
as stud dogs and brood bitches. This is important because their legacy carries the genetic
material for the current and future decades.
The information provided here outlines an approach that has been used successfully by the
author to reduce the prevalence of clinically expressed thyroid disease within susceptible
families or breeds.
EARLY THYROID DISEASE (THYROIDITIS)
COMPENSATORY AND CLINICAL 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 lets 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).
lgA deficiency / loss of smell (dysosmia) / loss
of taste / glycosuria / chronic active hepatitis / other endocrinopathies adrenal,
pancreatic, parathyroid
The Orthopedic Foundation for Animals (OFA) now has
a canine thyroid registry.
The following approved laboratories currently
can accept samples. Contact them for appropriate submission forms, sample handling
procedures, and laboratory service fees, before collecting the specimen.
Animal Health Diagnostic Laboratory
Endocrine Diagnostic Section
PO Box 30078
Lansing, MI 48909c7576
Diagnostic Laboratory
New York State College of Veterinary Medicine
Cornell University, Upper Tower Rd.
Ithaca, NY 14851
