Updated October 31, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
A dog who puts on weight without eating more, seems unusually tired and develops a thinner coat may have hypothyroidism. But these changes aren’t unique to thyroid disease. Arthritis, other hormonal disorders, chronic illness and medications can produce overlapping symptoms, so the diagnosis depends on more than a checklist.
What signs are common in hypothyroid dogs?
Because thyroid hormone influences metabolism, dogs may become less active, gain weight or seem less interested in cold-weather walks. Hair may thin symmetrically along the body or tail, and the coat may regrow slowly after clipping. Some dogs develop recurring skin or ear problems. These signs often appear gradually in middle-aged dogs rather than suddenly.
Can a blood test confirm it?
Total T4 is commonly used as a screening test, but a low result by itself is not proof of hypothyroidism. Concurrent illness and some drugs can lower total T4 even when thyroid function is adequate. Veterinarians may consider free T4, thyroid-stimulating hormone, medication history and the dog’s clinical picture before diagnosing and treating the condition.
What about an overactive thyroid?
Hyperthyroidism is much less common in dogs than in cats. In some cases, excessive thyroid hormone may be associated with a functioning thyroid tumor or unusual ingestion of thyroid-containing tissue. New lumps in the neck, unexplained changes in appetite or weight and other concerning signs deserve a veterinary examination.
What happens after diagnosis?
Dogs with confirmed primary hypothyroidism generally receive prescription levothyroxine and monitoring over time. Don’t start human thyroid tablets or increase a pet’s prescription because the dog still seems tired; the cause of ongoing symptoms may be unrelated.
See thyroid treatment and monitoring and how long clinical improvement may take.
Veterinary references
This article is for general education and is not a substitute for care from your own veterinarian. Learn about The Pet Vet and our editorial policy.