Canine Hypothyroidism

May 18, 2021

Written by: Joe Lyman, DVM, MS

Dogs may be affected by a variety of endocrine disorders. The one thought to be the most common is hypothyroidism. In fact, in a recent report by Banfield Pet Hospitals, 1 out of every 200 dogs they saw had hypothyroidism.  If we couple that with the estimates of the canine population in the U.S. being approximately 70 million, we can estimate the number of dogs in the U.S. suffering from hypothyroidism to be around 350,000.  Certain breeds may be more apt to develop hypothyroidism, but any breed can be affected.

A dog's thyroid gland is a bi-lobed gland located near the trachea that produces thyroid hormones, T3 and T4. These hormones have the role of regulating the body's metabolism and thus have an effect on every system/cell in the body. A negative feedback system fairly tightly regulates the production of these hormones. The part of the brain called the hypothalamus releases a substance called TRH (thyroid releasing hormone), which stimulates the dog's pituitary gland to produce TSH (thyroid-stimulating hormone). The TSH stimulates the thyroid gland to produce the thyroid hormones.  T4 is produced in the greatest amounts but ultimately is reduced to T3 by removing one iodine molecule. It is T3 which has the higher metabolic activity. The levels of T3 and T4 in the blood tell the hypothalamus and the pituitary if more or less of them is needed. For example, if the hormone levels fall, the hypothalamus and pituitary sense the decrease and release more TRH and TSH, respectively, which signals the thyroid gland to produce more T3 and T4. The reverse of this procedure occurs if the thyroid hormone levels exceed their normal range.

Hypothyroidism occurs when the level of circulating thyroid hormones is low. Typically, this occurs due to destruction or atrophy of the gland tissue caused by an attack by the body's immune system. If the thyroid gland tissue is lost, the gland's ability to produce adequate hormone levels is reduced.

As stated earlier, the thyroid hormones regulate the body's metabolism and thus have an effect on all systems of the body. Common symptoms seen in dogs with hypothyroidism include lethargy, weight gain, skin and hair coat problems, and poor tolerance of cold temperatures. 

A diagnosis of hypothyroidism is usually made by your veterinarian when any of these symptoms is found in combination with a low T4 level as measured in the blood. With such a complicated system used by the body to produce thyroid hormones, it is understandable why a definitive diagnosis of hypothyroidism may not be that simple. At times, other diagnostic tests such as measurement of T3, free T4, TSH, and thyroid antibodies may be necessary to confirm a suspected diagnosis of hypothyroidism.  Your veterinarian can help explain what these tests are used for. And to make things a little less straightforward, a situation that may be misleading is referred to as sick euthyroid syndrome.  This is when the dog's T4 level may be measured as low on a blood test, but the low T4 level is secondary to another disease or problem.

The good news is that the treatment for canine hypothyroidism is safe, effective, and easily achieved by administering a substance called levothyroxine. Levothyroxine is a synthetic version of the body's T4. This levothyroxine is usually in tablet form and essentially replaces the deficient T4 of the body.  Once therapy is started, the T4 blood levels return to normal fairly quickly.  Your veterinarian will likely recheck this blood level a short time (approximately one month) after starting therapy and periodically thereafter. The amount of levothyroxine given can be adjusted to reach a target range in the blood.  The next change usually seen is an increase in the activity and energy level of the dog. Changes in conditions of things like the skin and hair coat usually take a little longer to physically see but will gradually improve.  All of these improvements ultimately lead to a better quality of life for the dog.

Cats rarely suffer from hypothyroidism and more commonly develop hyperthyroidism which is characterized by having too much circulating thyroid hormone. As it turns out, what was once thought to be a fairly common problem in horses is actually not. Hypothyroidism in horses is rare. Those horses that are "easy keepers" are more likely suffering from a condition called equine metabolic syndrome.

As with all health issues with your pet, no matter which species, your veterinarian is always your best source of information and should be consulted if you are concerned about any of these conditions.

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