I.The Thyroid Gland

The thyroid gland is located in the neck and consists of two lobes located on either side of the trachea or windpipe. The thyroid gland uses iodine to make thyroid hormones. Most of the thyroid hormone circulating in the blood is thyroxine, or T₄. T₃ is another thyroid hormone that circulates in the blood. In the blood, most thyroid hormones are bound to proteins. To work, they are released from the proteins, taken up by tissues and activated. Thyroid hormones influence every organ in the body. Just some of the effects that thyroid hormones have include:

• Increasing overall metabolic rate


• Increasing heart rate


• Increasing the strength of contraction of the heart


• Formation of bone


• Normal development of a fetus, especially the nervous system and the skeleton


• Stimulating red blood cell production

II.What is Hypothyroidism?

Hypothyroidism is characterized by a decrease in production of thyroid hormone. The most common reasons for this are lymphocytic thyroiditis and thyroid atrophy. The cause of thyroid atrophy is unknown. Lymphocytic thyroiditis is an autoimmune disease, which means that the body’s own immune system attacks and destroys the gland. Rarely, hypothyroidism may be caused by a thyroid tumor.

Hypothyroidism is most common in middle-aged to older, large breed dogs. Commonly affected breeds include the Golden Retriever, Doberman Pinscher, Labrador Retriever, Cocker Spaniel, Irish Setter, Boxer, Miniature Schnauzer, Dachshund, Shetland Sheepdog, Airedale Terrier, Great Dane, and Poodle.

III.What to Watch for at Home – Symptoms

Because thyroid hormones affect every organ in the body, there can be many different symptoms. Usually the symptoms come on very gradually and are not noticed by owners. Frequently, symptoms are just attributed to aging. Some of the more commonly seen symptoms include:

• Lack of energy


• Depression


• Obesity even with appropriate food intake


• Facial expression described as “tragic”


• Hair loss that is symmetrically distributed on both sides of the body


• Dull hair coat


• Dry,scaly skin


• Superficial skin infection


• Ear Infections


• Weakness


• Megaesophagus (a dilated, poorly functioning esophagus)


• Facial paralysis


• In intact animals (not neutered), there may be reproductive difficulties such as infertility


• Rarely, dogs may become severely weak and depressed or go into a coma

Symmetrical Hair Loss on Both Sides of the Body with Hypothyroidism

Tragic Expression in a Dog with Hypothyroidism

IV.The Veterinary Visit – How to Tell if a Dog has Hypothyroidism

Diagnosis begins with the history and physical examination. Often these will cause a veterinarian to suspect that a dog is hypothyroid. The next step is blood work. This includes a complete blood count, biochemical profile and thyroid hormone level (T₄ level). The most common abnormalities on the blood work are anemia (low red blood cell count) and high cholesterol. The thyroid hormone level is almost always below the normal range. Unfortunately, other diseases can cause the thyroid hormone level to be low so usually another test must be run to confirm that a dog has hypothyroidism. Some of the other tests available are a free T₄ level, a T₃ level, a TSH response test and a baseline TSH level. TSH is thyroid stimulating hormone, a hormone that will cause the T₄ level to go up in normal dogs.

The TSH response test is the best test to confirm hypothyroidism. TSH is thyroid stimulating hormone. This is a hormone that should cause the thyroid (T₄) level to rise if the thyroid gland can still produce it. During this test, a dose of bovine TSH is given and the T₄ level is measured six hours later. If a dog is hypothyroid, its T₄ level will not go up.

If the TSH response test or other tests are not available, one other way to confirm a diagnosis of hypothyroidism is with a therapeutic trial. This means that a patient will be treated for several months to see if their symptoms resolve with therapy. If they do, then a presumptive diagnosis of hypothyroidism is made.

V.Treatment

Dogs that are hypothyroid are treated with synthetic thyroxine. This is a small pill that is given either once or twice daily. Symptoms may take a couple of months to resolve. Reproductive problems can take up to one year to resolve. Hypothyroidism cannot be cured. Dogs that are hypothyroid need to stay on medication for the rest of their lives. Once they are on medication, most dogs go on to lead normal lives.

Treatment is monitored by checking T₄ levels. The T₄ level is first checked six weeks after starting therapy and then on a yearly basis. Anytime the dose of medication is changed, a T₄ level should be checked four to six weeks after the change.