Hypothyroidism is the most common endocrine disorder in dogs, causing progressive metabolic slowing that impairs quality of life and is often misattributed to normal ageing.
Hypothyroid dogs experience a progressive decline in energy, mental alertness, and physical capacity that owners frequently attribute to ageing. The condition causes genuine welfare impairment: affected dogs are cold, lethargic, and mentally dull, and many develop painful skin conditions from the metabolic disruption. The degree of improvement seen with levothyroxine treatment is often striking, demonstrating the extent of pre-treatment suffering. Because the condition is easily treated at low cost, undiagnosed hypothyroidism represents preventable chronic welfare harm. Annual thyroid screening is warranted in predisposed breeds from middle age. The response to treatment is typically so dramatic that owners describe it as getting a different — much happier — dog.