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.
The thyroid gland is a 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 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 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 of Hypothyroidism in Dogs
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 may include:
Diagnosis of Hypothyroidism in Dogs
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:
Treatment of Hypothyroidism in Dogs
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.
Home Care for Hypothyroidism in Dogs
At home, administer all prescribed medication(s) and observe your dog closely for recurrence or improvement in the clinical signs. Over dosage 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.
Prevention of Hypothyroidism in Dogs
There are no effective ways to prevent hypothyroidism. However, once treatment begins, many symptoms resolve within several weeks.
InDepth 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:
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 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 baseline total thyroxine (T4) level is commonly measured. A thyroid level in the normal or upper T4 level generally indicates that the dog is not hyp0thyroid. 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 not using 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.
Your veterinarian may recommend additional tests to exclude or diagnose other conditions. They are selected on a case-by-case basis:
Therapy In-depth for Hypothyroidism in Dogs
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 and 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 in the medication schedule. Most common recommendations 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 measurement of T4 are necessary to ensure that the 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.