Canine hypothyroidism: What is it and how is it treated?

Dr. James Little
Director of professional services, Neogen


Dog_Sitting_blogWhen a dog’s body chemistry is off, the health effects can be significant.

This is especially true when it comes to the endocrine system, which regulates hormones. Although dogs may be affected by a range of endocrine issues, hypothyroidism is thought to be one of the most common. In fact, a 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. While it is true that certain breeds may be more apt to develop hypothyroidism, it is important to remember that any breed can be affected.

What is the thyroid?

Let’s start with the basics. The thyroid gland of the dog is a bi-lobed gland located near the trachea that produces the thyroid hormones known as T3 and T4. These hormones have the role of regulating the body’s metabolism and thus have an effect on every system and cell in the body. The production of these hormones is fairly tightly regulated by a negative feedback system, meaning the metabolic rate is correlated to the production of hormones.

A part of the brain called the hypothalamus releases a substance called thyroid releasing hormone (TRH) that stimulates the dog’s pituitary gland to then produce thyroid stimulating hormone (TSH). 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. Of the two, T3 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. This in turn signals the thyroid gland to produce more T3 and T4. The reverse of this procedure occurs if thyroid hormone levels exceed their normal range.

So what is hypothyroidism?

Hypothyroidism occurs when the level of circulating thyroid hormones is low. More specifically it is a lack of ability of the thyroid gland to produce these hormones.   Typically this happens as a result of destruction or atrophy of the gland tissue caused by an attack by the body’s own immune system. If the thyroid gland tissue is lost, the ability of the gland to produce adequate amounts of hormone is reduced.

As I mentioned 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 simple to obtain. 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 look for.  And, because life is never simple, hypothyroidism may be confused with sick euthyroid syndrome. This is a situation where the dog’s T4 level may be measured as being low on a blood test, but the low T4 level is secondary to another disease or problem.

How is it treated?

The good news is treatment for canine hypothyroidism is safe, effective, and easily achieved by the administration of a substance called levothyroxine. Levothyroxine is a synthetic version of the body’s own 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 (about a 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 such as 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, but that’s another issue for another day.

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.

For more information on Neogen’s ThyroKare thyroid products, please visit Neogen’s website or contact your veterinarian.


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