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Hypothroidism: a very treatable disease in dogs

Ever feel like your dog is getting heavier when you are not feeding him or her more? Does he or she have less energy than usual? These could be signs of an underactive, or "hypo" thyroid. The thyroid gland is located on the neck just below the larynx (Adam's apple) and produces thyroid hormone. Thyroid hormone is responsible for controlling metabolism. Too much of the hormone and the metabolism operates at too high of a rate. Too little and the metabolism slows down.

An underactive thyroid is a condition that is almost exclusive to dogs. Although cats can also get this condition it is not only rare, but usually secondary to treatment for a hyper- or over-active thyroid condition. Any dog can develop hypothyroidism (including mixed breed dogs) but there are certain breeds that are more at risk including golden retrievers, great Danes, Old English sheepdogs, boxers, English setters, Dalmations and cocker spaniels.

The most common cause of hypothyroidism is related to a condition called autoimmune thyroiditis. Autoimmune is a term that refers to any disease process that is initiated by a defect, or mistake in the body's immune system. Rather than only producing antibodies against foreign material (infections, cancer, etc), the body produces auto antibodies, or antibodies against the body's own tissues. In this case, the tissue is the thyroid gland. This attack of the thyroid gland by the immune system causes inflammation, or thyroiditis and eventually destroys the cells that produce thyroid hormone. As hormone decreases the metabolism slows down. Other less common causes of hypothyroidism are an iodine deficiency (iodine is essential in producing thyroid hormone), thyroid cancer or hypothyroid secondary to certain medications.

Symptoms

Symptoms of hypothyroidism are the result of a slowed metabolism. Lethargy, weight gain without increasing calories in the diet and alopecia (hair loss) are the most common.

Less common signs include a dry haircoat or pyoderma (an infection of the skin which mimics eczema). Rare signs include compromised heart function, facial palsy, seizures, central vestibular disease (a neurologic condition that affects the balance center in the brain leading to vertigo), coma and death. The good news is the majority of dogs with hypothyroidism are diagnosed long before these rare and more serious symptoms appear.

Diagnosis

Diagnosis of hypothyroidism is based on symptoms and bloodwork making it much less invasive and expensive than diagnosing other diseases in veterinary medicine. A measurement of T4 (active thyroid hormone) acts as a good screening test. If the T4 is low and the patient is showing signs of hypothyroidism this indicates the need for follow-up testing. Certain medications or other illnesses can decrease T4 levels, therefore some patients can have low T4 without hypothyroidism.

cTSH (canine Thyroid Stimulating Hormone) and a FT4D (free T4 by dialysis) are excellent follow up tests. cTSH is a hormone produced by the pituitary gland in the brain. cTSH stimulates the thyroid gland to produce more T4, or active thyroid hormone, and it works on a negative feedback cycle. This refers to a system where (in a healthy dog) when the thyroid hormone decreases it is recognized by the pituitary gland and the pituitary produces more cTSH. Once levels have risen the pituitary stops producing cTSH and, in turn, the thyroid stops producing T4.

A FT4D refers to a free T4 (active thyroid hormone) measured by dialysis. Measurement by dialysis is a more accurate way to measure active thyroid hormone and is affected less by certain medications or illnesses that can skew a T4 measurement. Although there are other tests that can be performed (thyroid sonograms, thyroid stress tests) a low FT4D and elevated cTSH is diagnostic in over 95 percent of cases.

Treatment

Once a diagnosis of hypothyroidism is made the treatment is simple. Levothyroxine (known in human medicine as Synthroid® or other trade names) is synthetic thyroid hormone that can be taken in pill form. It is lifelong medication but not cost prohibitive. Blood tests are needed to monitor treatment but are usually once every four to six months after the patient is regulated.

So, if your veterinarian is concerned with hypothyroidism in your dog do not fear. This is a very treatable disease.

Dr. Kearns has been in practice for 12 years and is pictured with his son Matthew, as well as the newest member of the family, Jasmine, a Labrador retriever.

Author: 
Matthew Kearns, DVM