Dog Hypothyroidism: Symptoms, Diagnosis & Treatment 2026
Hypothyroidism is the most common hormonal disorder in dogs. It occurs when the thyroid gland fails to produce sufficient thyroid hormone (T3 and T4), which regulates metabolism across virtually every body system, causing a gradual slowing of the dog’s entire physiology.
What Causes Hypothyroidism in Dogs?
Around 95% of cases result from lymphocytic thyroiditis (immune-mediated destruction of thyroid tissue) or idiopathic thyroid atrophy (gradual degeneration replaced by fat tissue). Medium to large breeds between 4–10 years are most commonly affected. Breeds with known predisposition: Golden Retrievers, Doberman Pinschers, Irish Setters, Cocker Spaniels, Miniature Schnauzers, Dachshunds, Boxers, and Great Danes.
Signs and Symptoms
- Weight gain without increased appetite — the most hallmark sign; metabolism slows and fat increases on a normal diet
- Lethargy and exercise intolerance — dog seems tired, slow to rise, less interested in activity
- Cold intolerance — seeks warm spots and shivers more than expected
- Skin and coat changes — dry, dull, or thickened skin; bilateral symmetrical hair loss especially on the trunk; “rat tail” appearance
- Facial changes — thickening of facial skin giving a “tragic facial expression”
- Reproductive issues — irregular heat cycles in females; reduced fertility in both sexes
- Neurological signs (advanced cases) — weakness, incoordination, seizures, or facial nerve paralysis
Diagnosis
Your vet starts with a baseline T4 (total thyroxine) blood test. A normal result rules out hypothyroidism; a low result is suggestive but not definitive — illness and certain medications (steroids, phenobarbital) can suppress T4 in dogs with normal thyroid function (euthyroid sick syndrome). For a more definitive diagnosis, vets also check Free T4 (by equilibrium dialysis) and TSH — elevated TSH alongside low T4 strongly indicates true hypothyroidism.
Treatment and Long-Term Management
Hypothyroidism is managed with daily synthetic levothyroxine (L-T4) — Soloxine or Thyro-Tabs — given once or twice daily. Within weeks, owners see returning energy and playfulness; coat regrows over 3–4 months; weight normalises gradually. This is a lifelong condition requiring permanent medication. Your vet will recheck thyroid levels at 4–8 weeks after starting treatment, then every 6–12 months. With proper management, hypothyroid dogs live completely normal, full lives.