The 4 Forms of Hypothyroidism in Dogs

While there are two main types of canine hypothyroidism, dogs are also subject to two rarer forms. Primary and secondary hypothyroidism account for most cases of dog thyroid disease. Neoplastic and congenital forms are far less common and often breed-specific.

Symptoms of Hypothyroidism

The thyroid gland is located in your dog's throat, on either side of the windpipe or trachea. The thyroid gland is responsible for the production of hormones that control metabolic function. When it malfunctions or under functions, inadequate hormones are produced. It is the most common reason for hormone imbalances in dogs.

Symptoms of hypothyroidism include skin abnormalities, hair loss, lethargy and weight gain. Any dog can suffer from hypothyroidism, but some breeds are more susceptible, including Golden Retrievers, Doberman Pincers, Dachshunds and Cocker Spaniels. Primary hypothyroidism-responsible for most cases of this dog disease-usually occurs between the ages of four and six.

Primary Hypothyroidism

Primary hypothyroidism accounts for at least 95 percent of all cases. In adult dogs, primary hypothyroidism is caused by the destruction of the pituitary gland. This is most often caused by two conditions: an immune-related condition known as lymphocytic thyroiditis and idiopathic atrophy, wherein the thyroid tissue is slowly replaced by fatty tissue.

Secondary Hypothyroidism

Secondary hypothyroidism occurs when a tumor encroaches on and replaces healthy thyroid tissue. Malignant thyroid tumors reduce hormone production in the dog. Cancer of this type is rare in dogs; tumors are more likely to be benign and cause hyperthyroidism or an overactive thyroid.

Rare Forms of Hypothyroidism

Other, rarer forms of hypothyroidism include an abnormal growth or destruction of thyroid tissue (neoplastic) caused by an iodine deficiency and congenital or juvenile onset hypothyroidism. Congenital hypothyroidism is an inherited condition that is seen in certain breeds including Giant Schnauzers and German Shepherd Dogs. Toy Fox Terriers also carry a gene for congenital hypothyroidism.

Diagnosis and Treatment

Hypothyroidism is diagnosed with a blood test. Samples are taken and tested for thyroid hormone concentrations in the blood. There are a variety of tests available for diagnosing hypothyroidism, and the results can be somewhat vague. Certain breeds such as the Greyhound have lower levels of certain marker hormones.

Primary hypothyroidism is very treatable. Daily medications are required to restore and maintain hormone levels and regular testing should be done on an ongoing basis to determine its effectiveness. The most common supplemental form of thyroid hormone replacement is L-thyroxine. It is available in liquid, tablet or chewable form, depending upon the brand and manufacturer.