Hypothyroidism in Dogs

Hypothyroidism usually develops in young to middle aged dogs (ie. 2 to 6 years old) secondaryHypothyroidism in Dogs to atrophy of the thyroid gland, that is, when the thyroid gland starts to waste away.  In some dogs with hypothyroidism the atrophy occurs secondary to lymphocytic thyroiditis, which is an immune-mediated disease that causes inflammation of the thyroid gland.  The cause of this disorder is not known, but it can lead to irreversible atrophy of the thyroid glands.

Clinical signs of hypothyroidism in dogs

Dogs with hypothyroidism can have many clinical signs.  The most common clinical sign of hypothyroidism in dogs  are dermatologic disease, lethargy, dull attitude, exercise intolerance, and a propensity to gain weight without eating more food.  The dermatologic changes with hypothyroidism in dogs are variable.  The most common dermatologic changes are symmetrical hair loss on their body that typically does not involve the head and legs.  The hair loss will sometimes only involve the tail (ie. “rat tail”).  Dogs with hypothyroidism also can have a greasy coat (seborrhea), or a dull, dry, and easily epilated hair coat (easy to pull out the dog’s hair).  Rarely, dogs with hypothyroidism will develop myxedema, which is thickening of the skin that often occurs in the forehead and facial region.  Other less common clinical signs of hypothyroidism in dogs are neurologic disorders, reproductive abnormalities, corneal dystrophy, cardiovascular disease, or gastrointestinal disease.

Diagnosis of hypothyroidism in dogs

The important thing for people to understand is that there is no one test, or series of blood tests that can definitively diagnose hypothyroidism in dogs.  On initial screening bloodwork, dogs with hypothyroidism will often have a high cholesterol level, high triglycerides, and a mild anemia.   The most common blood tests performed by veterinarians when evaluating dogs for hypothyroidism are a total T4, free T4, and TSH.   Typically a dog with hypothyroidism will have a low total T4 (protein bound and non-protein bound thyroid hormone), low free T4 (non-protein bound thyroid hormone), and a high TSH (pituitary thyroid stimulating hormone).  Although these are classic signs for a dog with hypothyroidism this does not confirm the diagnosis of hypothyroidism in dogs.  Dogs can be tested for lymphocytic thyroiditis with a blood test for antithyroglobulin antibodies.  If the dog has a positive test for antithyroglobulin antibodies it confirms lymphocytic thyroiditis, but it does not confirm that the dog has, or will ever develop, hypothyroidism.

Treatment of hypothyroidism in dogs

If there is a strong suspicion that a dog has hypothyroidism based on clinical signs and bloodwork, treatment with thyroid supplementation should be initiated with levothyroxine (synthetic thyroid T4).  If the diagnosis of hypothyroidism is correct all of the clinical and bloodwork abnormalities should resolve within 2 months.  Response to therapy should be based on clinical signs and measurement of blood T4 levels.  The prognosis for hypothyroidism in dogs with proper treatment is excellent.

Click here to learn about thyroid disease in cats.

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Dr Peter Nurre

Peter Nurre, DVM, Diplomate ACVIM (internal medicine) is a board- certified veterinary internal medicine specialist. His professional interests include internal medicine and cardiology.

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