Diagnosis
Diagnosis of hypothyroidism can be challenging as there is no single test that is completely accurate in identifying the truly hypothyroid dog. Minimizing false positives or negatives requires evaluation of a variety of parameters. The signalment and history can be the first step in ruling in or out a diagnosis; most hypothyroid dogs are middle age and of the mid to large breed size. Clinical signs that are suggestive of hypothyroidism generally are categorized as dermatologic, metabolic and neurologic.
Because hypothyroidism affects multiple organ systems, a complete workup is warranted including a complete blood count, biochemistry screen and urinalysis. A normocytic, normochromic nonregenerative anemia is a characteristic clinicopathologic finding, as well as hypercholesterolemia. Frequently a total thyroxine (TT4) is part of the in-clinic diagnostic profile, making it readily available and economical as an initial screen of thyroid function in a dog, but is useful only in identifying the euthyroid dog. In a dog with no evidence of non-thyroidal disease, a TT4 within reference range rules out hypothyroidism in most dogs. If TT4 is below the reference range, the dog may or may not be hypothyroid depending on whether it is affected by concurrent disease (euthyroid sick syndrome) or long-term drug therapy with highly protein-bound compounds that compete for TT4 binding sites. Further thyroid testing is necessary when TT4 is below the reference range.
The variety of thyroid function profiles available to veterinarians creates confusion as to which tests are essential and which are optional or less useful. The choice is dependent on the sensitivity, specificity and accuracy of the individual versus combined tests. The sensitivity of a test is the proportion that correlates with a true positive result. Specificity is the proportion that reflects true negative results. Accuracy is the proportion of results that are neither false positives nor false negatives.
The Society of Comparative Endocrinology developed a consensus statement on the diagnosis of hypothyroidism after evaluating a number of study reports of the relative sensitivity, specificity and accuracy of thyroid function tests (see Recommended Reading). The consensus of the panel of veterinary endocrinologists was that the most accurate single thyroid function test is the free T4 via equilibrium dialysis (fT4d) and when fT4d is evaluated in combination with canine TSH, results are highly specific. A low fT4d and a high cTSH result is diagnostic of hypothyroidism in most cases. Other endocrinologists recommend that the ideal profile includes three thyroid function tests: TT4, fT4d and cTSH. Evaluation of breed-specific reference ranges are of particular value in Greyhounds that typically have half the concentration of TT4 and fT4d as normal dogs.
Discordant results, particularly a normal cTSH in the face of a low TT4 or low fT4d must be evaluated in concert with other clinicopathologic data, the signalment, history (particularly of long-term drug therapy) and the clinical signs. With normal cTSH and low fT4d in a dog with a consistent history and with clinical signs suggestive of hypothyroidism, diagnosticians will usually rely on the fT4 result rather than the cTSH.
The utility of autoantibody analysis (anti-thyroglobulin, anti-T3, and anti-T4 autoantibodies) is in confirming lymphocytic thyroiditis as the cause of the thyroid dysfunction which occurs in 50-60 % of hypothyroid dogs. Autoantibody measurement has replaced evaluation of serum concentrations of T3, a common component of thyroid profiles of the past. Therapy does not differ with the etiology of the thyroid destruction so evaluation of autoantibodies is not necessary in most dogs. However, because hypothyroidism is a heritable disease, evaluation of autoantibodies is important in assessing the suitability of dogs of breeds considered at high risk as candidates for breeding and is a required test to receive OFA thyroid registry. It is unclear at this time whether the presence of autoantibodies can be used to predict if a dog will develop hypothyroidism.
Recent studies evaluating thyroid ultrasonography have shown promise in measuring thyroid volume and relative echogenicity to distinguish euthyroid glands from hypothyroid glands, and may prove to be the diagnostic tool veterinarians have been looking for to confirm hypothyroidism in cases of discordant thyroid profile results.
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