Updated November 1, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Older dogs often slow down, but it is worth looking beyond age when a once-active dog becomes unusually tired, gains weight or develops coat changes. Hypothyroidism can contribute to these signs, yet arthritis, medication effects and other illness can do the same. A careful history and appropriate testing matter more than assuming every senior dog needs thyroid supplements.
What may suggest low thyroid hormone
Dogs with hypothyroidism can show reduced activity, weight gain without an increased appetite, dry skin, hair thinning, recurrent infections and sensitivity to cold. Signs commonly develop gradually. Because many of them are nonspecific, a veterinarian also considers appetite, body condition, mobility and other health changes.
Testing requires interpretation
A total T4 blood test is often used initially, but low T4 alone does not confirm hypothyroidism. Other medical conditions and certain medicines can lower thyroid values. Free T4 and canine TSH may add useful evidence when interpreted with examination findings and other laboratory results. Some breeds naturally have lower thyroid values than average, which also affects interpretation.
What if the diagnosis is confirmed?
Prescription levothyroxine generally replaces missing hormone for the long term. The veterinarian determines dosing and follow-up timing and may recheck blood levels and symptoms. Energy can improve before coat quality or weight does. Giving more hormone because the dog still looks tired can be dangerous; unrelated problems may remain.
When to arrange an examination
Book an appointment for unexplained weight or coat changes, new lethargy, recurrent skin infections or trouble tolerating exercise. Collapse, serious breathing trouble or severe weakness needs urgent care rather than a routine thyroid check. See early hypothyroidism clues and tests.
Reference
Merck Veterinary Manual: Hypothyroidism in Animals, updated May 2025.
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.