One of the
simplest and least expensive tests to have done, which could possibly reduce or stop
seizures, is a test for low thyroid function or hypothyroidism. I have found from
experience that thyroid testing is very important in every case of canine epilepsy.
Dr. William Thomas, a board-certified neurologist, had this to say about thyroid testing:
1. Thyroid testing should be considered in any dog with recurrent seizures. Although the
relationship between hypothyroidism and recurrent seizures is unclear, thyroid testing is
relatively inexpensive and carries little risk to the patient.
2. In general, any dog that is diagnosed with hypothyroidism by appropriate testing should
be treated with thyroid replacement therapy. This applies to all dogs, whether or not they
suffer seizures. If the seizures improve with thyroid therapy, then great! If not, the
patient should still be treated because hypothyroidism can cause many other health
3. Thyroid replacement therapy is not indicated in dogs with seizures if the patient has
*not* been diagnosed with hypothyroidism. There are potential risks associated with any
therapy, so subjecting the patient to unnecessary treatment is rarely a good idea. On the
other hand, appropriate use of thyroid medication is one of the more safe and effective
treatments available in veterinary medicine.
4. Many illnesses as well as some drugs, including Phenobarbital, will lower the level of
certain thyroid hormones (for example, T4) without causing clinically significant
hypothyroidism. Thyroid replacement therapy is not indicated in these instances.
Measurement of a T4 concentration alone does not allow discrimination between true
hypothyroidism and the effects of drugs and illnesses. In these patients diagnosis of
hypothyroidism requires measurement of several different parameters of thyroid function
WB Thomas DVM
University of Tennessee
WITH DR. DODDS, DVM
Based on Dr.
Thomas' recommendations, I sat down with Dr. Jean Dodds for an overview on hypothyroidism
W. Jean Dodds, DVM, is a nationally and internationally recognized authority on blood and
immune disorders, thyroid disease and nutrition. Dr. Dodds is the president of Hemopet/Pet
Lifeline, the first national nonprofit animal blood bank and greyhound rescue/adoption
program serving North America. Hemopet also conducts nonprofit clinical research studies.
Dr. Dodds has
always been available to those with questions about hypothyroidism, blood and immune
disorders, and nutrition.
I hope all of this is helpful...
Joanne and Asia
Joanne Carson, Ph.D.
Dr. Joanne Carson: Dr.
Dodds, why should a dog with seizures have a thyroid panel done to test for low thyroid
function or hypothyroidism?
Dr. Jean Dodds: Because thyroid dysfunction can precipitate or aggravate existing seizure
disorders. The mechanism is unknown, but may relate to the vital role of thyroid hormones
in cellular metabolism of the central nervous system. In some cases the seizures are
related to thyroid dysfunction and when placed on appropriate thyroid medication the
seizures may no longer occur or are reduced in severity or frequency.
Dr. Carson: What are some of the classic symptoms of hypothyroidism?
Dr. Dodds: There are more than 50 different symptoms of hypothyroidism. Classical signs
include chronic skin disease, hair loss, weight gain, lethargy and slow metabolism,
although behavioral changes (aggression, hyperactivity, poor concentration,
passivity, phobias, anxiety or neuromuscular events such as seizures), and many other
signs of metabolic imbalances also can occur. This condition can mimic other disorders.
Dr. Carson: How many symptoms would a dog need to have to suspect hypothyroidism and have
a thyroid panel done?
Dr. Dodds: Only one symptom may be present to suspect low thyroid function. Seizures can
be a symptom of thyroid imbalance, which warrants performing a full thyroid panel.
Thyroid dysfunction in dogs is nearly always caused by low levels of thyroid hormones.
(i.e., HYPOthyroidism). The rare exception would be in thyroid cancer which may, but not
always, produce signs of HYPERthyroidism. As the thyroid gland controls all metabolic
functions, symptoms of hypothyroidism are many and can mimic a variety of other
conditions. Affected dogs can show only one, or several or even many classical symptoms,
which begin in subtle ways and then usually progress to more obvious signs as the disease
process that destroys thyroid tissue becomes more advanced. This can take months to years.
Occasionally symptoms disappear without treatment and the gland recovers.
Dr. Carson: What kind of thyroid tests are important and why?
Dr. Dodds: Because the diagnosis of hypothyroidism is often complicated, we need to run
complete thyroid profiles (i.e., T3, T4, free T3, free T4, T3 and T4 auto antibodies, plus
thyroglobulin auto antibody and TSH where indicated) rather than just one or two analytes
(e.g., T4, TSH, free T4) We need to look at the whole picture along with the clinical
history of the seizing animal, breed and family history for likelihood of seizure disorder
and thyroid disease. When I interpret profiles and make recommendations, I take all these
factors, including drugs being given, into consideration.
Dr. Carson: If a dog is diagnosed with low thyroid function, how soon after starting on
thyroid medication should thyroid re-testing be done and why?
After 6 to 8 weeks on thyroid supplementation given twice daily, at the appropriate dose
for the dog's age, breed and weight, the thyroid levels should be retested to determine
whether the current dose is correct This sample needs to be drawn 4 to 6 hours after the
morning pill was given. IMPORTANT: Correct therapeutic ranges at that time should be at
the upper 1/3 to 25% above the upper limits of the lab reference ranges (except for
geriatric dogs or sight hounds where mid-range is adequate).
Dr. Carson: If thyroid medication raises the metabolism, is it possible that Phenobarbital
and potassium bromide levels could change? If so, when should these levels be tested after
starting on thyroid supplementation?
Dr. Dodds: Yes, drug metabolism and drug clearance from blood and tissues can be altered
when body metabolic function is raised, such as with thyroid supplement. This could result
in raising or lowering Phenobarbital and potassium bromide levels in individual dogs,
depending upon their liver function and effectiveness of drug binding/clearance
mechanisms. Other drugs, chemical and toxic exposures, or stress situations present in
these dogs could also alter metabolism of thyroid and anticonvulsant medications.
Dr. Carson: I've heard that Phenobarbital can cause a low thyroid reading. Is this true
hypothyroidism and should a thyroid medication be used?
Dr. Dodds: Drug effects on thyroid binding proteins and thyroid levels do not produce true
hypothyroidism, and so treatment with thyroid supplement is unadvisable. However, many
dogs with seizure disorders have thyroid dysfunction independent of Phenobarbital use, and
so significantly lowered thyroid levels---especially of free T4---indicate the likely
benefits of thyroid treatment for assisting in seizure control and any other metabolic
symptoms of hypothyroidism that also may be present (e.g., chronic skin disease, weight
It is well known that Phenobarbital and some other drugs, such as potentiated sulfonamides
can affect blood levels of thyroid hormones. With the sulfonamides, after long-term use, a
clinically hypothyroid state can be produced. Regardless, it is typically the total T4
level that is most affected, with the free T4 being affected much less and temporarily,
depending to some extent on how it was measured.
Dr. Carson: Quite a few vets use only a TSH test to test for low thyroid function on epi's
taking Phenobarbital. Is this an accurate test?
Dr. Dodds: TSH is NOT a reliable assay when used by itself in the dog, and generally has
poor sensitivity and predictive value in the dog. This is in contrast to the use of
endogenous TSH assays in people which work very well.
Here are four very recent studies that support the fact that using endogenous canine TSH
as the sole assay is unreliable for diagnosing canine hypothyroidism.
JAVMA, Vol 212 1998 - Effect of time of sample collection on serum thyroid-stimulating
hormone concentrations in euthyroid and hypothyroid dogs. - Bruner, Scott-Moncrieff,
This study demonstrates the limitations of the current c-TSH assay. In the present study,
serum c-TSH was supportive of hypothyroidism in only 37% of the samples obtained from
spontaneously hypothyroid dogs. A normal serum
c-TSH concentration does not exclude hypothyroidism as a diagnosis. Current c-TSH assays
are often unnecessary for the diagnosis of hypothyroidism. (J Am Vet Assoc 1998,-
Pathol 1999 28:16-19 - Biological Variation of Canine Serum Thyrotropin (TSH)
Concentration - Iversen, Lundord-Jensen, Holer, Aaes
The present study was to estimate the between-dog, within-dog and analytical components of
variance for serum thyrotropin (TSH) in healthy dogs. The number of samples required to
determine the true mean value in a single dog
was 40. Comparing a single serum TSH measurement from an individual dog to the
conventional population-based reference range may be too insensitive to detect small but
important changes in the serum TSH level of that particular dog. (Vet Clin Pathol 28:16-19
JAVMA, Vol 211 No. 11 December 1, 1997- Measurement of serum total thyroxine,
triiodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of
hypothyroidism in dogs. - Peterson, Melian, and Nichols.
indicate that measurement of serum free 1 and TSH concentrations is useful for diagnosis
of hypothyroidism in dogs. About a quarter of the dogs with confirmed hypothyroidism,
however, will have serum
TSH concentrations within reference limits (J Am Vet Med Assoc 1997:211;1396-1402)
JAMVA, Vol 212. No. 3, February 1, 1998 - Comparison of serum concentrations of
thyroid-stimulating hormone in healthy dogs, hypothyroid dogs, and euthyroid dogs with
concurrent disease - Scott-Moncrieff, Nelson, Bruner, and Williams
had good specificity for use in the diagnosis of hypothyroidism in dogs. Because this
assay had low sensitivity, a diagnosis of hypothyroidism could not be excluded on the
basis of a serum cTSH concentration that was within the reference range. ( J Am Vet Med
Dr. Carson: Thank you Dr. Dodds. You have always been a wealth of information and always
accessible for questions.
Dodds, DVM resume'
Dr. Dodds was born on January 4, 1941. She received her D.M.V. in 1964 from the Ontario
Veterinary College. In 1965 she moved to the New York State Health Department is Albany
and began comparative studies of animals with inherited and acquired bleeding diseases.
This work continued full time until 1986 when she moved to Southern California to
establish Hemopet, the first nonprofit national blood bank program for animals.
From l965 to l986, she was a member of many national and international committees on
hematology, animal models of human disease, veterinary medicine, and laboratory animal
science. Dr. Dodds was a grantee of the National Heart, Lung, and Blood Institute (NIH)
and has over 150 research publications. She was formerly President of the Scientist's
Center for Animal Welfare, and Chairman of the Committee on Veterinary Medical Sciences
and Vice Chairman of the Institute of Laboratory Animal Resources, National Academy of
Sciences. In 1974 Dr. Dodds was selected as Outstanding Woman
Veterinarian of the Year, AVMA Annual Meeting, Denver, Colorado; in 1977 received the
Region I Award for Outstanding Service to the Veterinary Profession from the American
Animal Hospital Association, Cherry Hill, New Jersey; in 1978 and 1990 received the Gaines
Fido Award as Dogdom's Woman of the Year; and the Award of Merit in 1978 in Recognition of
Special Contributions to the Veterinary Profession from the American Animal Hospital
Association, Salt Lake City, Utah. In 1984 she was awarded the Centennial Medal from the
University of Pennsylvania School of Veterinary Medicine. In 1987 she was elected a
distinguished Practitioner of the National Academy of Practice in Veterinary Medicine. In
1994 she was given the Holistic Veterinarian of the Year Award from the American Holistic
Veterinary Medical Association, Orlando, Florida. She is a member of numerous professional
Today, Dr Dodds is actively expanding Hemopet's range of nonprofit services and
educational activities. On behalf of Hemopet, she consults in clinical pathology
nationally and internationally and regularly travels to teach animal health care
professionals, companion animal fanciers, and pet owners on hematology and blood banking,
immunology, endocrinology, (including thyroid) nutrition and holistic medicine. She is
also the editor of "Advances in Veterinary Sciences and Comparative Medicine"
for Academic Press.