Canine pseudohypothyroidism and covert hypothyroidism.

Primary hypothyroidism is perhaps the most common endocrinopathy of the dog. Unfortunately, a paradox exists. Many cases of mild to moderately severe hypothyroidism are unrecognized, while many euthyroid dogs are misdiagnosed as having hypothyroidism. Uncommon manifestations resulting from earlier stages of primary hypothyroidism, from secondary/tertiary hypothyroidism, or from a unique interplay between thyroid hormone deficiency and the age, sex, or breed affected can obscure the clinical suspicion of hypothyroidism. Conversely, nonthyroidal diseases and drug therapy can alter thyroid hormone metabolism in ways that mimic hypothyroidism and can lead to an erroneous diagnosis. Misinterpretation of clinical signs and laboratory findings are also causes for an incorrect diagnosis of hypothyroidism. The level of confidence in the diagnosis of hypothyroidism should be reflected as "suspected," "probable," and "definitive" hypothyroidism. Screening laboratory evaluation should be serum T4 and free T4 levels. The most reliable current evaluation for equivocal cases is the thyroid-stimulating hormone (TSH) stimulation test. Intramuscular or subcutaneous administration of TSH provides a greater separation between normal and abnormal post-TSH response. A definitive diagnosis of hypothyroidism in an adult dog is a retrospective assessment based on complete recovery after appropriate thyroid hormone replacement.