Hypothyroidism — underactive thyroid — is the most common endocrine disorder in dogs, causing a range of symptoms that significantly affect quality of life when undiagnosed or undertreated.
Hypothyroidism in dogs causes a slowly progressive welfare decline that owners often attribute to normal ageing. A dog with undiagnosed hypothyroidism experiences metabolic sluggishness — reduced energy, weight gain despite unchanged appetite, cold discomfort, skin and coat deterioration — for months to years before diagnosis. Neurological manifestations including weakness and facial paralysis cause additional welfare burdens. Because the decline is gradual, the baseline shifts imperceptibly and owners may not recognise the degree to which their dog's quality of life has deteriorated. Treatment with levothyroxine is remarkable in its effectiveness: most dogs show dramatic improvement in energy, coat quality, and demeanour within weeks. This makes hypothyroidism a high-priority condition to recognise and test for in dogs showing any compatible signs.