Hypothyroidism is the most common endocrine disease in dogs, causing weight gain, lethargy, skin disease, and a range of metabolic effects that significantly impair quality of life when undiagnosed or undertreated.
Hypothyroid dogs experience profound metabolic slowing that affects their entire quality of life. The weight gain occurs even on normal food intake, leading owners and sometimes veterinarians to focus on diet rather than endocrine disease. The lethargy reduces the playful engagement that is central to dogs' wellbeing and owners' bonds with their pets. Skin changes including hair loss, dry coat, and thickening cause discomfort and predispose to secondary infections. The cold intolerance means hypothyroid dogs are uncomfortable in temperatures that normal dogs tolerate easily. The transformation after appropriate levothyroxine supplementation is often dramatic - dogs return to previously active personalities within weeks, confirming the degree of impairment caused by untreated thyroid deficiency.