Canine hypothyroidism causes weight gain, lethargy, skin disease, and cognitive changes. Lifelong levothyroxine supplementation restores excellent welfare in virtually all affected dogs.
Hypothyroidism develops insidiously, and the gradual onset of signs means it is often unrecognized for months to years. During this undiagnosed period, dogs experience chronic welfare harm: persistent lethargy, weight gain that increases joint load and reduces mobility, skin disease causing discomfort and self-trauma, and sometimes neurological signs including vestibular dysfunction or facial nerve paralysis.
The characteristic behavioral changes — reduced activity, increased sleep, reduced interest in play — are frequently misattributed to aging rather than disease. Owners accepting these changes as inevitable aging deny their dogs welfare-improving treatment that could restore energy, normal weight, and coat quality.
Levothyroxine replacement therapy is dramatically welfare-restorative. Most dogs show significant improvement within 4-8 weeks of starting therapy — owners describe their dogs becoming 'young again'. The treatment is lifelong but inexpensive, and monitoring with T4 levels every 6 months ensures dose remains appropriate as needs change. Dogs treated for hypothyroidism have normal life expectancy and excellent quality of life.