Canine Hypothyroidism

By Dr. Mikel Delgado, PhD

What is hypothyroidism?

Hypothyroidism is a condition where the thyroid gland is underactive and does not produce enough hormones to meet the needs of the body.

The thyroid gland

Endocrine glands are organs that secrete hormones into the bloodstream to affect various functions of the body. The thyroid gland is one of these organs which regulates metabolism.  The thyroid gland has two lobes that are located on either side of the trachea in the neck. 

Another endocrine gland, the pituitary gland, controls the activity of the thyroid gland through the hormone TSH (thyroid-stimulating hormone). 

When stimulated by the pituitary gland, the thyroid releases two hormones: T3 - triiodothyronine, and T4 - thyroxine, which converts into T3 in the cells of the body. Once released into the blood, T3 and T4 regulate how fast the cells of the body work, impacting heart and muscle function, as well as digestion. Proper functioning of the thyroid gland is critical to your dog’s health.

What can go wrong with the thyroid gland?

The two most common issues with the thyroid gland are that it is overactive (hyperthyroid) or underactive (hypothyroidism). Hyperthyroidism is rare in dogs, but dogs can become hypothyroid in middle age. The true prevalence of hypothyroid in dogs is unknown, and although it is considered relatively common, many veterinarians believe it is overdiagnosed.

What causes hypothyroidism?

Lymphocytic thyroiditis is an autoimmune condition believed to be the most common cause of hypothyroidism. It is a heritable condition in which the immune system of the body treats the thyroid gland as foreign, resulting in destruction of thyroid tissue and reduced functioning of the thyroid gland.

Hypothyroidism can also be caused by idiopathic (of unknown cause) thyroid atrophy, where thyroid tissue is replaced by fat. Idiopathic thyroid atrophy is not considered heritable.

Note: Although anecdotally, some have claimed a connection between canine hypothyroidism and vaccination, no studies have found an association between hypothyroidism and vaccination in dogs.

Signs of hypothyroidism

Hypothyroidism usually appears in adult, often middle-aged dogs. Some of the most common signs of hypothyroidism include weight gain, lethargy, and issues with the coat (e.g., dull coat, shedding, balding). Some dogs will also seek out warmth, and may have chronic skin or ear infections. Breeding dogs that are hypothyroid may lack interest in mating (decreased libido), or may have decreased fertility .

How do you test for hypothyroidism?

If your dog has signs of hypothyroidism, your veterinarian will want to do some diagnostic tests, including tests that will rule out other causes of the signs you are observing. A CBC (complete blood count), blood chemistry, and urinalysis may be recommended in addition to specific thyroid tests.

The most common test for hypothyroidism is a Total T4 which measures the levels of T4 that are either bound to proteins or freely circulating in the sample. 

If thyroxine is low in the Total T4 test, then a follow up test is required to make a definitive diagnosis. Follow-up tests include the free T4 test, which only measures thyroxine that is not bound to proteins (or is “free”), and a full thyroid panel. The Free T4 is more accurate, but is also more expensive and can take longer to get results, which is why the Total T4 is often done first. 

Tests for TSH (thyroid-stimulating hormone) levels measure the amount of this hormone being produced by the pituitary gland. Due to the interaction between the pituitary and thyroid glands, when the thyroid gland is underactive, the pituitary gland ramps up TSH to try to stimulate the thyroid gland to make more T4. TSH results should always be interpreted simultaneously with other T4 tests.

Finally, your veterinarian may test your dog’s TgAA (Thyroglobulin Autoantibody) levels. Autoantibodies are antibodies that attack the individual’s own cells, and are often an indicator of autoimmune disorders. These antibodies are released into the bloodstream when a dog has lymphocytic thyroiditis, or in some cases of older dogs, when thyroid cancer is present. A low level of TgAA does not rule out thyroid disease.

What is the treatment for hypothyroidism?

Hypothyroidism in dogs is treated with medication, a thyroid replacement hormone called Levothyroxine. Frequent blood tests may be required to ensure that the level of medication is appropriate for your dog, and the dose may need adjusting over time. If dogs receive too much of the thyroid replacement hormone, they can become hyperthyroid. Your dog will need this medication for the rest of their life.

Some medications can interact with thyroid replacement hormones. Your veterinarian can advise you how to balance treatment for hypothyroidism with any other conditions your dog may have. 

Hypothyroid and your breeding program

Some breeds are predisposed to lymphocytic thyroiditis, including Golden Retrievers, Doberman Pinschers, Cocker Spaniels, and Dachshunds. At this time, there is no available DNA test for lymphocytic thyroiditis, and the mode of inheritance is unclear. It is likely a polygenic condition (influenced by multiple genes). If lymphocytic thyroiditis is detected in your breeding dogs, they should not be bred again.

What is the prognosis for dogs with hypothyroidism?

Hypothyroidism is treatable, but there is no cure. Your dog will be on medication for life, but medication failure is uncommon. In general, as long as dogs are receiving the appropriate amount of medication, their prognosis is good.

Resources

VCA Hypothyroidism in Dogs

VCA Thyroid Hormone Testing in Dogs

Cornell University College of Veterinary Medicine Canine Thyroid Testing

Bell, J. S. (2003). Hereditary hypothyroidism: Understanding the disease process. In Tufts’ Canine and Feline Breeding and Genetics Conference.

Bergmann, M., Freisl, M., Hartmann, K., Speck, S., Truyen, U., Zablotski, Y., ... & Wehner, A. (2021). Antibody response to canine parvovirus vaccination in dogs with hypothyroidism treated with levothyroxine. Vaccines, 9(2), 180.

Mooney, C. T. (2011). Canine hypothyroidism: a review of aetiology and diagnosis. New Zealand Veterinary Journal, 59(3), 105-114.

Scott‐Moncrieff, J. C., Glickman, N. W., Glickman, L. T., & HogenEsch, H. (2006). Lack of association between repeated vaccination and thyroiditis in laboratory Beagles. Journal of Veterinary Internal Medicine, 20(4), 818-821.

Spence, S. (2022). Canine hypothyroidism: avoiding over diagnosing the condition. In Practice, 44(2), 68-75. https://bvajournals.onlinelibrary.wiley.com/doi/full/10.1002/inpr.74