Hypothyroidism in Dogs

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The thyroid gland has a number of different functions, but it is most well-known for its role in regulating metabolism by producing thyroid hormones. Hypothyroidism is the condition that occurs when not enough of these hormones are produced. Although the onset of clinical signs is variable, hypothyroidism most commonly develops in middle-aged and senior dogs, and is the most common hormone imbalance of dogs. Breeds that are predisposed to this condition include the Doberman Pinscher, Golden Retriever, Irish Setter, Great Dane, Dachshund, Boxer, Miniature Schnauzer, Cocker Spaniel, and the Airedale.

What are the Symptoms?

Thyroid hormones are needed for normal cellular metabolic function. A deficiency of thyroid hormones affects the metabolic functions of all organ systems. As a result, the symptoms are usually variable and non-specific. There is not a specific symptom that is diagnostic for hypothyroidism. There are, however, several symptoms that when combined together make the veterinarian suspicious of the animal having the disease.

Symptoms seen with this disease are:

  • High blood cholesterol
  • Anemia
  • Lethargy/mental dullness
  • Hair loss
  • Weight gain/obesity
  • Dry hair coat/excessive shedding
  • Hyperpigmentation of the skin
  • Cold intolerance
  • Slow heart rate

If you notice these or any other changes in your dog, please contact your veterinarian immediately.

How is hypothyroidism diagnosed?

There are several different tests used to diagnose hypothyroidism in the dog. The test chosen will depend on the symptoms. The most common test run is the baseline T4 test. A blood sample is drawn and tested by radioimmunoassay to determine the level of T4 thyroid hormone in the bloodstream. The T4 hormone is produced only in the thyroid gland and dogs with a failure of the thyroid gland will have a lowered level of this hormone. However, there are other conditions that can cause a lowering of T4 so if this screening test is positive for hypothyroidism, another more specific test is often done to confirm the diagnosis.

How is hypothyroidism treated?

Hypothyroidism in dogs is easily treated. Treatment consists of placing the dog on a daily dose of synthetic thyroid hormone called thyroxine (levothyroxine), which they will need to be on for the rest of their life. The dose and frequency of administration of this drug varies depending on the severity of the disease and the individual response of the animal to the drug. A dog is usually placed on the standard dose for his weight and then a blood sample is drawn three months later to check his response and to adjust the dose accordingly.

Once therapy is started, the dog will need to be on treatment for the rest of his life. Usually after treatment is started, the majority of symptoms will resolve. The T4 test is run yearly to monitor the thyroid hormone level and medication adjustments are made if needed.