DIAGNOSIS AND MANAGEMENT OF CANINE THYROID DISEASE

W. Jean Dodds, D.V.M.
HEMOPET, 938 Stanford Street, Santa Monica, CA 90403
reprinted by permission of Dr. W. Jean Doggs
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Although thyroid dysfunction is the most frequently recognized endocrine disorder of the dog, a definitive diagnosis may be difficult to establish. Clinical signs of thyroid dysfunction mimic symptoms resulting from other causes and interpretation of results of thyroid function tests can be problematical. Most canine thyroid disease is the result of autoimmune thyroiditis, which is a familial disorder of inherited predisposition similar to human Hashimoto's disease. Therefore, the most complete approach to thyroid testing should include assays for thyroid autoantibodies.

1. Baseline Thyroid Profiles

Because of the difficulties inherent to diagnosing thyroid disease, a complete baseline thyroid panel is advisable and should include measurements of total T4, total T3, free T4, free T3, and circulating T4 and T3 autoantibodies. This type of profile can be applied not only to clinical patients suspected of having thyroid disease, but also can be used
for genetic screening of apparently healthy relatives to evaluate their fitness for breeding. A bitch with circulating antithyroid antibodies can pass these along to the puppies transplacentally as well as via the colostrum. Furthermore, dogs with circulating antithyroid autoantibodies may eventually develop clinical symptoms of thyroid disease and/or be susceptible to other autoimmune diseases. Thyroid prescreening is thus very
important for selecting potential breeding stock.

2. Genetic Screening for Thyroid Disease

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 to 14 months in males and during the first anestrous period for females following their maiden heat). As the female sexual cycle is quiescent during anestrus, any influence of sex hormones on baseline thyroid function will be avoided.
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 will be more reliable when they are tested in anestrus. Once the initial thyroid profile is obtained, dogs and bitches should be rechecked on an annual basis to assess their thyroid function and overall health. This allows for early treatment, where indicated, to avoid the appearance or advancement of clinical signs associated with hypothyroidism.

For optimal health, young dogs under 15 to 18 months of age should have thyroid baseline levels in the middle to upper half of adult normal ranges. This is because puppies and adolescent dogs are still growing and maturing. Similarly, in older animals above 8 or 9 years of age, body functions slow down so that baseline thyroid levels may be in the lower third of the range in euthyroid individuals. For healthy young adults used for performance or breeding, optimum thyroid function should be at least at the mid-point of
the laboratory normal ranges. Lower levels may be indicative of the early stages of thyroiditis among relatives of dog families previously documented to have thyroid disease.

3. Diagnosing Difficult or Equivocal Cases

Some dogs with typical clinical signs of hypothyroidism have circulating levels of thyroid hormones within the normal range. Most of these patients will improve clinically when given thyroid medication, because blood levels of thyroid hormones may not reflect cellular and tissue thyroid levels. A 6 to 8 week clinical trial of thyroid supplementation given twice daily is safe and appropriate for such patients, and is followed by rechecking the complete thyroid profile 4 to 6 hours after the morning pill. Response to thyroid therapy is considered an appropriate justification for continuing to prescribe thyroid hormone. The usual therapeutic dosage is 0.1 mg (100 ug) per 4.5 Kg (10 lbs.) BID.

Once the appropriate dose of thyroid supplement is established, annual retesting is recommended unless the dog exhibits any sign of illness in the intervening period.

If an animal is receiving thyroid supplementation and the original diagnosis is questioned, the clinician may wish to reevaluate the patient. Whenever thyroid therapy is discontinued, retesting is performed after 4 or preferably 6 weeks, because it takes at least a month for the animal's own pituitary-thyroid axis to be restored to full capacity.