Hypothyroidism in Dogs: Signs, Diagnosis, and Long-Term Management

The Underactive Thyroid in Dogs

Hypothyroidism — insufficient production of thyroid hormones (T3 and T4) by the thyroid gland — is one of the most common endocrine disorders in dogs. Unlike cats, who predominantly develop hyperthyroidism (overactive thyroid), dogs develop hypothyroidism. The majority of cases result from immune-mediated destruction of thyroid tissue (lymphocytic thyroiditis) or idiopathic atrophy. Mid-size to large breeds are overrepresented, with Golden Retrievers, Doberman Pinschers, Irish Setters, Great Danes, and Boxers among the most commonly affected.

Recognizing the Signs

Hypothyroidism is called the "great imitator" because its signs are diffuse and non-specific. Classic findings: weight gain without increased appetite, lethargy and exercise intolerance, cold intolerance (seeking warmth, reluctant to go outdoors in cool weather), coat changes (bilateral symmetric hair loss, particularly on the trunk and tail, with a dull, dry coat and thickened skin), and mental dullness. Less common but recognized signs include peripheral neuropathy, vestibular disease, and cardiac abnormalities. The collection of signs — particularly the weight gain plus symmetric hair loss plus lethargy combination — is the classic presentation.

Diagnosis

Screening uses a baseline total T4 (TT4) measurement. Low TT4 combined with compatible clinical signs is suggestive of hypothyroidism, but TT4 alone can be suppressed by non-thyroidal illness ("euthyroid sick syndrome"). Confirmation typically uses the free T4 by equilibrium dialysis (fT4 ED) — more expensive but more specific — along with TSH (thyroid stimulating hormone). High TSH combined with low fT4 is the most reliable diagnostic combination. Testing should be performed when the dog is not acutely ill, as illness suppresses thyroid values.

Treatment and Monitoring

Treatment is oral levothyroxine (T4 replacement) given once or twice daily, usually for life. Response is gratifying: most hypothyroid dogs improve significantly within 4-8 weeks of appropriate treatment, with energy returning, weight normalizing, and coat improving. Monitoring with TT4 measurement 4-6 hours after a dose is performed at 4-8 weeks after starting, then every 6-12 months. Over- and under-dosing both have consequences; monitoring is not optional. Supplementation with desiccated thyroid products not prescribed by a veterinarian should be avoided.