This is a disease of thyroid deficiency. The thyroid gland sits on the throat below the larynx. Its function is to produce the hormones thyroxine (T4) and triiodothyronine (T3), which control the rate of metabolism. Thus, dogs with hypothyroidism have metabolic rates below normal. Hypothyroidism, in most cases, is caused by autoimmune thyroiditis (also called lymphocytic thyroiditis), which results in destruction of thyroid tissue. Autoimmune thyroiditis is known to be an inherited disease. Idiopathic thyroid gland atrophy is a rare cause of hypothyroidism. The cause of both types of thyroid gland atrophy is unknown, but environmental and dietary factors are possible contributing factors.
The disease occurs most often in middle-aged dogs of medium and large breeds, but has been seen in virtually every breed and in mixed breeds as well. The breeds most commonly affected are the Golden Retriever, Doberman Pinscher, Irish Setter, Miniature Schnauzer, Dachshund, Shetland Sheepdog, Cocker Spaniel, Airedale Terrier, Labrador Retriever, Greyhound, Scottish Deerhound, and others. Hypothyroidism is the most common endocrine skin disease in dogs-but it is still less common than other skin diseases. Coat and skin changes are bilateral and symmetrical. A typical sign is poor hair regrowth, which is most noticeable after the dog has been clipped.
It is also common to see hair loss that involves the front of the neck down to the chest, the sides of the body, the backs of the thighs, and the top of the tail. The hair is excessively dry and brittle, and falls out easily. The exposed skin is dry, thick, puffy, and darkly pigmented. Some dogs develop secondary seborrhea.
Other signs of hypothyroidism include weight gain, intolerance to cold, a slow heart rate, absence of heat cycles, lethargy, and a variety of nonspecific symptoms that could be due to a number of other diseases. Hypothyroid dogs may develop blepharitis, corneal ulcers, deafness, adult-onset megaesophagus, chronic constipation, and anemia. Hypothyroidism has been found in association with dilated cardiomyopathy, strokes, coronary artery disease (rare in dogs), von Willebrand's disease, and myasthenia gravis. At least two-thirds of hypothyroid dogs have high serum cholesterol levels. Finding elevated serum cholesterol on routine blood screening warrants a workup for hypothyroidism. Behavior changes, including aggression, have also been noted in hypothyroid dogs, particularly German Shepherd Dogs.
The recommended blood test for screening purposes is the total T4. This test is indicated for dogs who have findings suggestive of hypothyroidism on physical examination. A normal T4 is fairly conclusive evidence that the dog does not have hypothyroidism. However, a low-normal or below-normal level does not mean the dog is hypothyroid, because concentrations below normal are common for many reasons other than hypothyroidism.
To avoid overdiagnosing and overtreating the disease, it is important to confirm the significance of a low T4 using a more accurate thyroid function test, such as the FT4 by equilibrium dialysis. Other blood tests are also available for diagnosing hypothyroidism. One is an assay for thyroglobulin autoantibodies; these autoantibodies are present in about 50 percent of dogs with autoimmune thyroiditis. This test must be sent to a special laboratory for analysis.
Treatment: Hypothyroidism is permanent, but can be effectively treated with daily or twice-daily thyroid hormone replacement using synthetic L-thyroxine (L-T4). The initial dose is based on the dog's weight. This should be adjusted for individual circumstances. Monitoring is done by physical examination and measuring the total T4. This must be done frequently, particularly early in the course of treatment. Hair loss and other signs of hypothyroidism usually reverse with treatment. A few dogs will need T3 supplementation as well.
The Orthopedic Foundation for Animals (OFA) maintains a hypothyroid registry to identify dogs as normal for breeding (see appendix D). A certificate and breed registry number is issued to all dogs found to be normal at 12 months of age, based on the results of FT4, cTSH, and thyroid autoantibody screening by an OFA-approved laboratory. Screening and registration is of value for dogs at risk of inherited hypothyroidism. If the test is positive, these dogs should not be used for breeding.
Michigan State University also has a thyroid registry and keeps statistics on thyroid function in dog breeds.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.