Hypothyroidism in Dogs: The Complete Guide
The thyroid gland regulates metabolism. When it under-produces thyroid hormone, every body system slows down — weight creeps up, energy disappears, the coat dulls. Because the changes are gradual and non-specific, hypothyroidism is frequently attributed to ageing until a blood test reveals the truth. The diagnosis is actually welcome: it's one of the most treatable hormonal conditions in dogs.
Symptoms
- Weight gain without increased food intake
- Lethargy — less interested in walks, sleeping more
- Cold intolerance — seeking warmth, shivering easily
- Dull, dry coat and skin
- Symmetrical hair loss — often on the trunk, without itching (this distinguishes it from allergic hair loss)
- The "tragic face" — subtle thickening of facial soft tissue giving a droopy, sad expression
- Recurrent skin and ear infections
- Bradycardia in severe or long-standing cases
Causes
- Lymphocytic thyroiditis (~50 %): autoimmune destruction of thyroid tissue
- Idiopathic atrophy (~50 %): progressive replacement of thyroid tissue with fat, cause unknown
Predisposed breeds: Golden Retrievers, Dobermanns, Irish Setters, Boxers, Cocker Spaniels, Airedale Terriers. Middle-aged dogs (4–10 years) most commonly affected.
Diagnosis
A baseline T4 (total thyroxine) screen is the starting point, but it has limitations — illness, fasting, and many medications can suppress T4 in euthyroid dogs (sick euthyroid syndrome). A free T4 by equilibrium dialysis (fT4 ED) and TSH together give a much more reliable picture. Clinical signs + laboratory results = diagnosis.
Treatment
Oral levothyroxine (synthetic T4), typically twice daily. Dose by body weight; re-check T4 and TSH 4–6 weeks after starting, then every 6–12 months once stable.
Most dogs show striking improvement within weeks: energy returns, weight drops, coat quality improves. Treatment is lifelong — but inexpensive, well-tolerated, and highly effective.
Log thyroid test results and medication doses in Purzi. When the vet runs a follow-up test, having the full timeline — previous values, when the dose was adjusted, how your dog responded — makes the consultation far more productive.