Monitoring Subclinical Thyroid Disease

Author: Sharon M Albright, DVM, CCRT

Hypothyroidism, a common hormone disorder in dogs, is defined by deficient thyroid hormone levels and is usually caused by auto-immune destruction of the thyroid gland. Hypothyroidism is often diagnosed in middle-aged, large-breed dogs, but can occur in any dog. Clinical signs are caused by slowed cell metabolism and include lethargy, weight gain, and skin and coat changes such as dry coat and hair loss. Veterinarians diagnose hypothyroidism based on clinical signs and measurement of various hormone levels in the blood. Treatment is often successful but requires life-long administration of an oral hormone supplement.

Most dogs with subclinical thyroiditis continue to have thyroid testing abnormalities long-term.

Thyroid Hormone Tests

Total thyroxine (TT4): Thyroxine is the primary hormone produced by the thyroid gland. This test measures the amount of thyroxine bound to proteins and floating freely in the bloodstream.

Free thyroxine (FT4): This test measures only thyroxine that is floating freely in the bloodstream and is less affected by medications or other illnesses.

Total tri-iodothyronine (TT3): Another form of thyroid hormone created by modification of T4 outside of the thyroid gland.

Thyroglobulin autoantibodies (TgAA): A measurement of antibodies that bind to thyroglobulin, a protein involved in the production of T3 and T4. These autoantibodies indicate inflammation within the thyroid gland.

Thyroid stimulation hormone (TSH): TSH is produced by the pituitary gland and stimulates the thyroid gland to increase hormone production.

A clear diagnosis of hypothyroidism is made when a dog shows typical clinical signs and thyroid testing reveals low hormone levels, elevated TSH, and elevated TgAA. But what if the diagnosis is not so clear? 

Some dogs have a syndrome called subclinical thyroiditis. These dogs have elevated TgAA levels, indicating inflammation in the thyroid gland, but their hormone and TSH levels remain normal. Are these dogs at risk of progressing to clinical hypothyroidism? If so, how great is that risk?

A previous study showed that hormone levels remained stable in dogs with subclinical thyroiditis for one year. To assess the long-term risk of disease development, researchers at Michigan State University retested dogs with subclinical thyroiditis two to nine years after their initial testing with funding from AKC Canine Health Foundation (CHF) Grant 02659: Breed Specific Reference Ranges for Canine Thyroid Testing. They measured hormone levels and collected clinical histories from 125 dogs representing 31 different breeds.

Results recently published in the American Journal of Veterinary Research confirm that the majority of dogs with subclinical thyroiditis continue to have long-term thyroid testing abnormalities. Approximately one third of them will eventually progress to clinical hypothyroidism that requires treatment. Only a small number of them will convert to normal thyroid function. This underscores the importance of monitoring dogs with subclinical thyroiditis at least once a year or more often if clinical signs of hypothyroidism develop.

Studies like this refine our understanding of hypothyroidism - the most overdiagnosed hormonal disease in dogs. They provide scientifically sound information that guides our diagnostic and treatment decisions.  Because thyroid hormone levels are influenced by disease outside of the thyroid gland, the administration of certain medications, and a dog’s breed, it is important for veterinarians and dog owners to have this evidence. CHF and its donors remain committed to funding studies that provide this evidence and have a real impact on the health of our beloved dogs. Learn more at

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