OVERVIEW

Hypothyroidism is a disorder of the thyroid gland, which is the butterfly-shaped glandular organ located in the neck just below the voicebox. This gland is responsible for producing and secreting thyroid hormone (thyroxine), which affects nearly all body systems. Thyroxine is the hormone that is primarily responsible for regulating the metabolic rate of many different tissues in the body. In hypothyroidism, not enough thyroxine is produced, which causes the metabolism of these tissues to slow down.

Hypothyroidism is the most common endocrine (hormonal) disorder in dogs and often develops between the ages of 4 to 10 years. It usually affects castrated male and spayed female dogs. A variety of dog breeds are afflicted with this condition, such as the golden retriever, Doberman pinscher, greyhound, Irish setter, dachshund, miniature schnauzer, Great Dane, poodle, and boxer. The disease also occurs in mix breed dogs and many other breeds.

Hypothyroidism can be divided into primary and secondary causes. Most cases are primary, which means that there is destruction of the thyroid gland from inflammation, degeneration, or infiltration with tumor. Inflammation of the thyroid gland is thought to be caused by the dog’s own immune system. Secondary hypothyroidism develops when some other influence causes the thyroid gland to produce less thyroxine. Examples include diseases of the areas of the brain that regulate thyroid gland activity, destruction of the thyroid gland from radiation therapy, surgical removal of the thyroid glands, or the administration of certain medications that affect thyroid gland activity.

CAUSES

Lymphocytic thyroiditis is an inflammation of the thyroid gland that arises when the immune system attacks the gland by mistake. There is an increased prevalence of this disease in the English setter, boxer, Brittany spaniel, dalmation and other purebred dogs.

Idiopathic follicular atrophy is degeneration of the thyroid gland without evidence of inflammation. The cause of this condition is unknown, but it may reflect the end stage of thyroiditis.

Thyroid tumors (neoplasia) are an uncommon cause of hypothyroidism. Unless the tumor affects both lobes of the thyroid and the gland is thoroughly destroyed, hormone output usually remains normal.

Dietary iodine deficiency is a rare cause of hypothyroidism. Most commercial dog foods contain adequate levels of iodine.

Cretinism/dwarfism is a rare birth defect in dogs that arises because the hypothalamus in the brain does not produce enough thyroid-releasing hormone or the pituitary gland at the base of the brain does not produce enough thyroid-stimulating hormone (TSH). This condition has been seen in giant schnauzers and boxers.

Tumors and other diseases of the pituitary gland may also result in a deficiency of thyroid-stimulating hormone.

Surgical removal of the thyroid gland (thyroidectomy) results in an inability to produce thyroxine. Thyroidectomy is most often performed to remove a thyroid tumor in dogs.

Radiation therapy and radioactive iodine treatments for thyroid tumors may also cause hypothyroidism.

The use of some medications, particularly the sulfa-containing antibiotics, affects the activity of the thyroid glands, and may result in low out put of thyroxine. This form of hypothyroidism may be transient, because the gland may recover once the drug is stopped.

A deficiency of thyroid hormone affects the metabolic function of many organ systems. As a result, the clinical signs are usually variable, non-specific, and slow to develop. Although there is no one symptom that is diagnostic, several combined signs may make your veterinarian more suspicious. Symptoms may include:

Lethargy, lack of interest in play, increased amounts of sleeping
Depression, mental dullness
Exercise intolerance (tires easily)
Weight gain (without an apparent increase in appetite) and obesity
Cold intolerance – seeks out warm places to lie down, low body temperature
Slow heart rate (bradycardia)
Infertility
Constipation
Diarrhea
Chronic skin disorders, such as dry skin, thinning of the hair coat, excessive hair loss
Possibly other neurologic signs
Possibly other hormonal abnormalities

Hypothyroidism is not always a simple, straightforward disease to diagnose. Various tests are available to diagnose the condition and a combination of tests may be required. Proper diagnosis also includes a thorough history, documentation of clinical signs, a thorough physical examination, and diagnostic tests to assess various organ functions, including thyroid function. A diagnostic work-up may include the following:

Complete blood count (CBC)
Biochemical profile
Urinalysis
Thyroxine (T4) level, free-T4 level
Canine TSH test
Thoracic (chest) and abdominal radiographs (X-rays) in certain cases

Fortunately, hypothyroidism is easily treated and involves the administration of daily doses of synthetic thyroxine. Once treatment is started, it must usually be continued for the rest of the dog’s life.

At home, administer all prescribed medication(s) and observe your dog closely for recurrence or improvement in the clinical signs. Overdosage with thyroid medication causes signs of hyperactivity and weight loss. Your veterinarian may schedule periodic blood tests to measure the levels of thyroid hormone in order to monitor whether the dosage being given is adequate.

There are no effective ways to prevent hypothyroidism. However, once treatment begins, many symptoms resolve within several weeks.

INFORMATION IN-DEPTH

Thyroid hormone affects many different cells in the body and deficiency of this hormone leads to a wide variety of symptoms. Because the history, clinical signs and presentation of dogs with hypothyroidism are so variable, there are other illnesses/symptoms that are often considered when establishing a definitive diagnosis. These diseases include:

Hyperadrenocorticism (Cushing’s disease) is an endocrine disorder that arises when the body makes too much cortisone hormone. Dermatologic (skin) and hair coat problems with this disease are common and may appear similar to those of hypothyroidism.

Growth hormone deficiency is a rare hormonal disorder that commonly is associated with hair loss and occurs in middle-aged small to medium-sized dogs.

Other causes of reproductive dysfunction, such as infertility, failure to cycle, prolonged bleeding associated with heat, and lack of libido (sex drive) must also be considered.

Neuromuscular (nerve and muscle) diseases that affect the nerves of the head, the larynx, and other peripheral nerves around the body must be ruled out.

Other causes of obesity and lethargy must be ruled out.

VETERINARY CARE IN-DEPTH

Veterinary care includes using diagnostic tests to confirm the presence of hypothyroidism and any secondary changes in the body, as well as instituting appropriate therapeutic and monitoring procedures.

DIAGNOSIS IN-DEPTH

Certain diagnostic tests must be performed to make a definitive diagnose of hypothyroidism and exclude other disease processes that may cause similar symptoms. A complete history and a thorough physical examination are essential in reaching the correct diagnosis. The following diagnostic tests are often recommended:

A complete blood count (CBC) may reveal a chronic anemia.

A biochemical profile may show the presence of high cholesterol and other fats in the blood. Sometimes liver tests and creatine kinase (CK), a muscle enzyme, are also elevated.

A urinalysis is usually within normal limits.

Thoracic (chest) and abdominal radiographs (X-rays) may be normal, except for evidence of obesity, but they are important to rule out other causes of similar clinical signs.

Thyroid function tests are necessary to confirm the presence of hypothyroidism. There are three common tests used to assess thyroid function in the dog. These include:

A baseline total thyroxine (T4) level is commonly measured. Generally, a normal T4 level indicates that the dog is not hypothyroid. A low T4 level does not always confirm hypothyroidism, however, because other illnesses and certain drugs may affect the thyroid gland and suppress T4 levels. With a low T4 value, further testing may be needed.

A free-T4 (fT4) assay is also commonly performed. With this assay, thyroxine is separated from its carrier proteins, which eliminates the effects of other illnesses and most drugs on the levels of T4 measured. A low value on this test is more indicative of hypothyroidism.

Measurement of canine thyroid-stimulating hormone (cTSH) may also be performed. This test is usually elevated in dogs with hypothyroidism. An elevation occurs in this test because the pituitary gland detects that the levels of thyroxine circulating in the blood are low, and it secretes more TSH that travels to the thyroid gland to stimulate it to make more thyroxine.

Many veterinarians prefer to measure T4, fT4 and cTSH all at the same time, and then assess how their results compare. A low T4 or fT4 in the presence of an elevated TSH confirms the diagnosis of hypothyroidism in most cases.

A positive response to therapy with sodium levothyroxine also helps to confirm the diagnosis.
Your veterinarian may recommend additional tests to exclude or diagnose other conditions. They are selected on a case-by-case basis:

A thyroid gland biopsy may be recommended, especially in those cases where cancer is likely.

A radioactive nuclear scan may be used to assess the function of the thyroid gland. Hyperactive glands take up more radioactivity, where as hypothyroid glands take up less.

Measurement of antibodies to T4 and thyroglobulin (a thyroxin-binding protein) may be done. Elevations in these antibodies often indicate immune-mediated inflammation of the thyroid gland.

Other tests that may be run depend on the clinical signs and what other diseases are being considered as other potential causes of those signs.

Thyroid hormone supplementation is indicated for the treatment of hypothyroidism, and it is administered for the life of the individual.

Synthetic (man-made) levothyroxine(T4) is the drug of choice for treating hypothyroidism. The dosage and frequency of administration are determined by your veterinarian, and are usually based on the dog’s body weight. Levothyroxine is usually given twice daily in dogs.

There are both brand name and generic levothyroxine products available commercially. Use of a brand name product rather than a generic product is usually preferred. There appears to be great differences in how dogs respond to some generic products, and some dogs do not seem to respond very well to the generic drugs. Once a dog is stabilized on a particular thyroid medication, it is also better if the dog remains on that product consistently, rather than bouncing from one product to another.

FOLLOW-UP

Optimal treatment requires a combination of home and professional veterinary care. At home it is important to administer all medication exactly as prescribed by your veterinarian.

With appropriate therapy, most of the clinical alterations associated with primary hypothyroidism are reversible. Generally, an improvement in attitude and activity are seen within a couple of weeks, and hair coat and skin changes improve within four to six weeks of the initiation of therapy.

Most dogs tolerate thyroid supplementation very well; however, overdosage is associated with signs of hyperthyroidism. Watch the dog closely for signs of anxiety, hyperactivity, pacing and restlessness, excessive drinking and/or urination, weight loss and diarrhea, and report these signs to your veterinarian promptly.

It is important to follow-up with regularly scheduled visits to your veterinarian so that both your dog’s clinical signs and thyroid concentration in the blood can be monitored. Generally, the first follow-up examination is four to eight weeks after the start of therapy. T4 levels are often measured four to six hours after the morning pill is given or just prior to the evening pill. Adjustments in the dosage of medication are then recommended depending upon the results of these tests. Additional recheck visits are then scheduled based upon the test results, changes in clinical signs, and any alterations in the medication schedule.

Periodically throughout the dog’s life, repeated measurement of T4 are necessary to ensure that the hypothyroidism remains well controlled. Additional testing depends on the clinical course of the illness.

The prognosis for reversal and control of primary hypothyroidism is very good. Hypothyroidism associated with disorders of the hypothalamus and pituitary gland, as well as tumors of the thyroid gland, has a poorer prognosis.