Several drugs can affect thyroid function test results in humans and eventually lead to an erroneous evaluation of thyroid function. These medications can alter the synthesis, secretion, transport, or metabolism of thyroid hormones. Some drugs also directly inhibit the hypothalamic-pituitary-thyroid axis. The effects of drugs on thyroid function in dogs have long been underestimated and have most likely contributed to the overdiagnosis of hypothyroidism in this species. This manuscript 1st reviews pertinent thyroid physiology followed by an overview of drugs for which the effects on canine thyroid function have been studied. The effects of glucocorticoids, propranolol, sulfonamides, phenobarbital, potassium bromide, and nonsteroidal anti-inflammatory agents (NSAIDs) on canine thyroid function are summarized here. Knowledge of the potential effect of these medications on thyroid function should contribute to a more reliable interpretation of thyroid function test results in dogs.