Diagnosis of hypothyroidism is typically based on a combination of clinical signs and laboratory findings. The condition is commonly overdiagnosed in dogs, especially because many medical conditions can be associated with reduced circulating concentrations of thyroid hormone. Since available diagnostic tests lack high sensitivity and specificity, its definitive diagnosis can be difficult. In the past 20 years, development of newer diagnostic tests has enhanced the ability to differentiate canine hypothyroidism from other conditions. The most commonly used criteria to diagnose hypothyroidism are decreased total thyroxine (TT4) or free thyroxine (FT4) in combination with increased thyroid-stimulating hormone (TSH) concentration.