The Low Thyroid Seizure Connection
Epil-K9 talks to Jean Dodds, DVM
Epil-K9: Dr. Dodds, why should a dog with seizures have a thyroid panel done to test for low thyroid function or hypothyroidism?
Dr. 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 become less frequent.
Epil-K9: 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.
Epil-K9 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.
Epil-K9: 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 (e.g. T3, T4, free T3, free T4, T3 and T4 autoantibodies, plus thyroglobulin autoantibody 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 seizuring 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.
Epil-K9: If a dog is diagnosed with low thyroid function, how soon after starting on thyroid medication should thyroid re-testing be done and why?
Dr. Dodds: 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
IMPORTANT: Correct theraputic 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 sighthounds where mid-range is adequate).
Epil-K9: If thyroid medication raises the metabolism, is it possible that phenobarbitol 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 phenobarbitol 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.
Epil-K9: I've heard that phenobarbitol 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 phenobarbitol 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 gain, lethargy).
It is well known that phenobarbital and some other drugs, such as potentiated sulfonamides can effect 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.
Epil-K9: Quite a few vets use only a TSH test to test for low thyroid function on epi's taking phenobarbitol. 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, Williams.
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,- 212:1572-1575)
- Vet Clin Pathol 1999
28:16-19 - Bioogical 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 mall but important changes in the serum TSH level of that particular dog. (Vet Clin Pathol 28:16-19 1999
- 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.
CONCLUSION: Results 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
CONCLUSION: cTSH assay 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 Assoc l998:212:387-391)
Contact Dr. Dodds at 949-252-8455 FAX 949-252-0224 Wed. & Thurs. at HEMOPET or at home/office 310-828-4804, FAX 310-828-8251. Mon., Tues. & Fri. The best time to reach her is from 10 AM - 6PM PST Forms for testing
Page last update: 12/13/2011