The most common sample received by our endocrine testing laboratory is subm
itted for the diagnosis of hypothyroidism in a dog. The current tests most
frequently employed in our laboratory for thyroid evaluation in dogs are to
tal T4, free T4 by dialysis, and canine TSH measurement. Each test has stre
ngths and weaknesses and suffers from the possibility of both false positiv
e and false negative results. This article provides a working description o
f each test and an approach to interpretation of results. Other tests that
are less commonly used are also discussed. Examples of interpretation of te
st results in individual hypothyroid-suspect dogs are presented for illustr
ation.