Updated September 25, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: The most common thyroid disorder in dogs is hypothyroidism, an underactive thyroid. It usually develops in middle-aged dogs and causes weight gain without eating more, tiredness, cold intolerance, hair loss or a thin coat, and recurring skin or ear infections. Overactive thyroid (hyperthyroidism) is rare in dogs, unlike cats. Because many illnesses can lower thyroid levels, diagnosis needs careful testing.
Signs of hypothyroidism
- Weight gain without increased appetite
- Lethargy and less interest in exercise
- Seeking warmth
- Hair loss on the body and tail (“rat tail”), dull or dry coat, slow regrowth after clipping
- Darkened skin
- Recurring skin and ear infections
- Slow heart rate
Diagnosis
- Total T4 as a screening test
- Free T4 and TSH to confirm
- Other illnesses and some medications (like phenobarbital or steroids) can lower T4 without true hypothyroidism, so vets interpret results carefully
Treatment
Daily thyroid hormone replacement (levothyroxine), with blood level checks. See thyroid medication for dogs.
Hyperthyroidism in dogs
Rare, usually caused by a thyroid tumor or, in some reports, by raw diets or treats containing thyroid tissue.
Sources
- Merck Veterinary Manual: Hypothyroidism in dogs
- Scott-Moncrieff JC. Clinical signs and concurrent diseases of hypothyroidism in dogs and cats. Vet Clin North Am Small Anim Pract. 2007
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