THYROID DISEASE
AUTOIMMUNE THYROID DISEASE: 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.
COMMON PROBLEM OF PUREBRED DOGS
W. Jean Dodds DVM
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 Neuromuscular Problems Dermatologic Diseases Reproductive Disorders Cardiac Abnormalities Gastrointestinal Disorders Hematological Disorders Ocular Diseases Other Associated Disorders 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 Diagnostic Laboratory
Alterations in Cellular Metabolism
weakness / stiffness / laryngeal paralysis / facial
paralysis / tragic expression / knuckling or dragging feet /
muscle wasting / megaesophagus / head tilt / drooping eyelids
lethargy / mental dullness / exercise intolerance /
neurologic signs polyneuropathy / seizures / weight gain /
cold intolerance / mood swings hyperexcitability / stunted
growth / chronic infections
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
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
slow heart rate (bradycardia) / cardiac arrhythmias /
cardiomyopathys
constipation / diarrhea / vomiting
bleeding / bone marrow failure / low red blood cells / low
white blood cells / low platelets
corneal lipid deposits / corneal ulceration / uveitis
Keratococonjunctivitis / sicca or dry eye / infections of
eyelid glands (Meibomian gland)
lgA deficiency / loss of smell (dysosmia) / loss of taste /
glycosuria / chronic active hepatitis / other endocrinopathies
adrenal, pancreatic, parathyroid
Endocrine Diagnostic Section
PO Box 30078
Lansing, MI 48909c7576
New York State College of Veterinary Medicine
Cornell University, Upper Tower Rd.
Ithaca, NY 14851