Updated November 1, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
A dog who seems tired, gains weight without eating more, or develops a thinning coat may have hypothyroidism. But those signs are common to many illnesses, and age alone can change a dog’s energy and skin. Diagnosing thyroid disease takes more than checking one number on a blood test.
Changes owners often notice first
Dogs with low thyroid hormone may become less interested in exercise, gain weight despite an unchanged appetite, develop dry or dull hair, lose hair in a relatively symmetric pattern, or experience recurrent skin and ear problems. Some seek warm places more often. The change is usually gradual rather than a sudden collapse. Not every dog shows all of these signs, and some dogs are overweight or have skin disease for entirely different reasons.
Why one low T4 result is not enough
Total T4 is a useful screening test, but levels can fall when a dog has another illness or is taking certain medicines. Breed and age also influence interpretation. Veterinarians consider clinical signs, examination findings, routine laboratory results and, when indicated, additional tests such as free T4 by equilibrium dialysis and canine TSH. Treating a dog simply because a single result is borderline can lead to unnecessary medication.
What treatment involves
When hypothyroidism is confirmed, veterinary-prescribed levothyroxine replaces the missing hormone. The veterinarian selects the schedule and rechecks blood levels and clinical response, rather than adjusting treatment by appearance alone. Energy may improve before hair and body condition do. Follow-up testing remains important even after a dog seems better. Do not share human thyroid tablets or use over-the-counter thyroid supplements.
When to call sooner
Contact a veterinarian if your dog’s energy changes markedly, it develops painful skin infection, or it is losing weight rapidly or breathing abnormally. Those are not symptoms to assume are caused by an underactive thyroid. For older animals, read how thyroid problems overlap with aging.
Preparing for the visit
Bring a list of medications, supplements and recent illnesses. Record weight changes, appetite, exercise tolerance, coat changes, and when each started. Ask which test results support the diagnosis and when to recheck them.
Reference
Merck Veterinary Manual: Hypothyroidism in Animals (updated 2025); 2023 AAHA Selected Endocrinopathies Guidelines.
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.