Die canine Hypothyreose – eine diagnostische Herausforderung?

Abstract

Hypothyroidism is one of the most common endocrinopathies in dogs. Clinical symptoms and hematological and biochemical parameters lead to a first suspicion. To confirm diagnosis can be challenging, however. Determination of total serum T4 concentration is accepted as the primary screening test for the disease, and low serum T4 concentrations are intuitively suggestive of hypothyroidism. However it is well known that low T4 concentrations are frequently encountered in euthyroid dogs with various nonthyroidal diseases and in dogs receiving certain pharmacologic agents. Since assessment of endogenous TSH (canine TSH) using current canine TSH assays shows normal values in a high percentage of hypothyroid dogs (up to 40%), its diagnostic value is only limited. The TSH-stimulation test can still be recognized as the gold standard for the diagnosis of hypothyroidism in dogs. Determination of circulating T4 concentration before and 6 hours after the administration of exogenous TSH (recombinant human TSH, Thyrogen ® ) provides an assessment of the functional reserve capacity of the thyroid gland with minimal change in post-TSH T4 concentration, compared with the basal concentration, expected in dogs with hypothyroidism. Also this test can be influenced by nonthyroidal illness and by medications known to affect thyroid function. This suppressing influence seems to be less pronounced using a higher dose of TSH. Therefore, to improve the discriminatory power of the TSH stimulation test to differentiate between euthyroid-sick and primary hypothyroidism, the higher dose should be used in cases in which testing cannot be delayed. More recently, ultrasonography and scintigraphy have been used for the diagnosis of primary hypothyroidism. Using ultrasonography, a sensitivity of 98% was reported if size and echogenicity of the gland were combined. However, specificity was as low as 77% and care must be taken when measuring the gland because of a relatively high interobserver variability. Preliminary results using scintigraphy to discriminate between hypothyroid and non-thyroidal illness seem promising. However, due to its limited availability and the high expenditure, to date it cannot be used in clinical practice. Therefore, at present, if test results are obscure and a final diagnosis cannot be made, it is recommended to repeat the tests at a later time point.