The most common sample received by our endocrine testing laboratory is submitted for the diagnosis of hypothyroidism in a dog. The current tests most frequently employed in our laboratory for thyroid evaluation in dogs are total T4, free T4 by dialysis, and canine TSH measurement. Each test has strengths and weaknesses and suffers from the possibility of both false positive and false negative results. This article provides a working description of each test and an approach to interpretation of results. Other tests that are less commonly used are also discussed. Examples of interpretation of test results in individual hypothyroid-suspect dogs are presented for illustration.