An In-Depth Guide to Hypothyroidism in Dogs

Overview of Canine Hypothyroidism

Hypothyroidism, also referred to as cretinism, is a clinical disorder resulting from decreased blood levels of thyroid hormones (thyroxine [T4] and triiodothyronine [T3]) in dogs.

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 2 to 9 years, most commonly at middle age. 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 mixed-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.

Hypothyroidism in Dogs: Symptoms to What to Watch For

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 of hypothyroidism in dogs may include:

• Lethargy, lack of interest in play, increased amounts of sleeping
• Depression, mental dullness
• Exercise intolerance
• 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, persistent anestrus (lack of heat cycles)
• Constipation
• Diarrhea
• Chronic skin disorders, such as dry skin (seborrhea), thinning of the hair coat, excessive hair loss
• Possibly other neurologic signs such as weakness, trouble walking, seizures, head tilt, facial drooping, or changes in bark
• Possibly other hormonal abnormalities including dwarfism, delayed dental eruption, mental retardation, and/or short limbs. These signs are uncommon.

In-Depth Information on Hypothyroidism in Dogs

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). 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 like those of hypothyroidism.
• Growth hormone deficiency is a rare hormonal disorder that is associated with hair loss and occurs in middle-aged small to medium-sized dogs.
• Reproductive dysfunction, such as infertility, failure to cycle, prolonged bleeding associated with heat, and lack of libido (sex drive).
• Neuromuscular 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 as well.

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 on Hypothyroidism in Dogs

Certain diagnostic tests must be performed to make a definitive diagnosis 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 chronic anemia.
• A biochemical profile may show the presence of high cholesterol and other fats in the blood. Sometimes liver enzyme 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 dogs. These include:

– A baseline total thyroxine (T4) level is commonly measured. A thyroid level in the normal or upper T4 level generally indicates that the dog does not have hypothyroidism. 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. If clinical symptoms suggest hypothyroidism and the T4 level is normal, additional diagnostic testing is recommended.

– Serum Free T4 by Equilibrium Dialysis (FT4ED) measures the free available T4 in the blood. 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. This is the single most accurate test for diagnosing hypothyroidism.

However, it does not use a routine screening test because it is more time intensive and expensive to run. A low value on this test is more indicative of hypothyroidism.

– cTSH – 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.
• 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, whereas 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 potential causes of those signs.

Hypothyroidism in Dogs: Medication and Treatment

• Thyroid hormone supplementation is indicated for the treatment of hypothyroidism, and it is administered for the life of the individual.
• Synthetic (man-made) levothyroxine sodium (synthetic 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. A liquid formulation is available in Europe.
• Dogs with concurrent diseases such as heart disease, kidney failure, liver disease, diabetes, and/or hypoadrenocorticism (Addison’s disease) should be given a dose reduced by 25 to 50% and gradually increased.
• 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.
• Liothyronine sodium (synthetic T3) is rarely given and is generally given only when a dog with hypothyroidism has not responded ideally to treatment with levothyroxine sodium (synthetic T4) supplementation. Combining products is not recommended.

Home Care for Dogs with Hypothyroidism

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, over-dosage is associated with signs of hyperthyroidism. Watch the dog closely for signs of anxiety, hyperactivity, pacing, restlessness, excessive drinking and/or urination, weight loss, and diarrhea, and report these signs to your veterinarian promptly.

Follow-Up Care for Hypothyroidism in Dogs

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. 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 to the medication schedule. Most common recommendation is to check the thyroid level every 8 weeks for 6 – 8 months then every 6 to 12 months.

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

Prognosis for Hypothyroidism in Dogs

The prognosis for reversal and control of primary hypothyroidism in dogs 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.