This article was published by Akita Alumni in April of 1996 DIAGNOSIS AND MANAGEMENT OF CANINE THYROID DISEASE W. Jean Dodds, DVM Although thyroid dysfunction is the most frequently recognized endocrine disorder of the dog, it is sometimes difficult to make a definitive diagnosis. Many clinical signs of thyroid dysfunction mimic symptoms resulting from other causes so that recognition of the problem and interpretation of results of thyroid function tests can be problematical. The majority of canine thyroid disease is due to autoimmune thyroiditis, which is a familial disorder of inherited predisposition similar to that of 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, the use of 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, any dog having 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. In fact, genetic screening of intact females for other disorders such as von Willebrand’s disease, hip dysplasia, inherited eye diseases, and wellness or reproductive checkups is best scheduled during 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. Generation of annual test results provides comparisons that permit early recognition of developing thyroid dysfunction. This allows for early treatment, where indicated, to avoid the appearance of 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, and require higher levels of thyroid hormones to promote development. Similarly, in older animals above 8 or 9 years of age, body functions slow down and so baseline thyroid levels may be in the lower third of the range in the euthyroid individual. 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 The difficulty in accurately diagnosing some cases of thyroid disease is compounded by the fact that some patients 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. This is because the circulating blood levels of thyroid hormones may not reflect the cellular and tissue levels of these same hormones. 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 along with annual checkups to adjust dosages as appropriate.   In the event that an animal is already receiving thyroid supplementation and the basis for the original diagnosis is unclear, the clinician may elect to discontinue therapy and retest. In such cases, or when thyroid therapy is discontinued for any other reason, retesting is performed after another 4 or preferably 6 weeks. This is because it takes at least a month for the animal’s own pituitary-thyroid axis to be restored to full productive capacity after cessation of thyroid therapy. To perform thyroid retesting before this period has elapsed would yield relatively low blood levels of thyroid hormones which would bias interpretation of test results in favor of hypothyroidism.   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.