Hypothyroidism in Dogs

What is the thyroid gland and what does it do?

The thyroid gland is one of the most important glands in the body. It is located in the neck near the trachea or “windpipe” and has two lobes, one on each side of the trachea. The thyroid gland is controlled by the pituitary gland, which is located at the base of the brain.

“If the thyroid is overactive (hyperthyroidism), the body’s metabolism is elevated. If it is underactive (hypothyroidism), the metabolism slows down.”

The thyroid gland regulates the body’s metabolic rate. If the thyroid is overactive (hyperthyroidism), the body’s metabolism is elevated. If it is underactive (hypothyroidism), the metabolism slows down.

What causes hypothyroidism?

In the dog, hypothyroidism is usually caused by one of two diseases: lymphocytic thyroiditis or idiopathic thyroid gland atrophy. The former disease is the most common cause of hypothyroidism and is thought to be an immune-mediated disease. This means that the immune system decides that the thyroid is abnormal or foreign and attacks it. It is unclear why this occurs. In idiopathic thyroid gland atrophy, normal thyroid tissue is replaced by fat tissue. This condition is also poorly understood.

These two causes of hypothyroidism account for more than 95% of the cases in dogs. The other five percent are due to rare diseases, including cancer of the thyroid gland.

What are the clinical signs?

“When the metabolic rate slows down, virtually every organ in the body is affected.”

When the metabolic rate slows down, virtually every organ in the body is affected. Most dogs with hypothyroidism have one or more of the following symptoms:

Some dogs also have other abnormalities such as:

• Thickening of the facial skin so they have a “tragic facial expression”
• Abnormal function of nerves causing non-painful lameness, dragging of feet, lack of coordination, and a head tilt
• Loss of libido and infertility in intact males
• Lack of heat periods, infertility, and abortion in females
• Fat deposits in the corneas of the eyes
• Keratoconjunctivitis sicca (KCS) or “dry eye” due to lack of proper tear production

How is it diagnosed?

The most common screening test is a Total Thyroxin (TT₄) level. This is a measurement of the main thyroid hormone in a blood sample. A low level of total thyroxin, along with the presence of  clinical signs is suggestive of hypothyroidism. Definitive diagnosis is then made by performing a Free T4 by Equilibrium Dialysis (Free T₄ by ED) and/or a “thyroid panel” that assesses the levels of multiple forms of thyroxin. If this test is low, then your dog has hypothyroidism. Some pets will have a low TT₄ and normal Free T₄ by ED. These dogs do not have hypothyroidism. Additional tests may be necessary based on your pet’s condition.

Can it be treated?

“Hypothyroidism is treatable but not curable.”

Hypothyroidism is treatable but not curable. It is treated with oral administration of thyroid replacement hormone. This drug must be given for the rest of the dog’s life.

How is the proper dose determined?

All dogs are started on a standard dose of thyroid replacement hormone, based on the dog’s weight. After one month of treatment, a blood sample is taken to verify that the thyroid hormone levels are normal.

“Since the dog’s tolerance of the thyroid replacement hormone may change over time, the dose may need to periodically adjusted.”

Since the dog’s tolerance of the thyroid replacement hormone may change over time, the dose may need to periodically adjusted. Thus, it is usually necessary to retest TT₄ levels every six months. Close communication with your veterinarian is necessary in order to ensure that your dog is neither over nor under-dosed.

What happens if the medication is overdosed?

Signs of hyperthyroidism can result. These include hyperactivity, lack of sleep, weight loss, and an increase in water consumption.

Since the thyroid gland is such an important gland, it is extremely important that hypothyroidism be properly controlled. If you have any questions about this complicated disease, or if you are concerned about your pet’s response to treatment, please don’t hesitate to contact your veterinarian.