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.