Every day, veterinarians are asked to see patients that may have thyroid hormone deficiency. Hypothyroidism can manifest in multiple ways. Therefore, the results of diagnostic tests used to distinguish normal from sick dogs can be confusing. In the course, we will review normal thyroid physiology, etiology of canine hypothyroidism, the role of thyroid function testing, and specific thyroid function tests. We will also discuss new information on dosing and monitoring patients once they have been diagnosed with hypothyroidism.