The hormone is produced by the Thyroid Gland which is located in the neck below the voice box.
Hypothyroidism is considered as the most common hormonal abnormality in dogs. Most cases of Hypothyroidism have been observed in dogs between 4-10 years of age. High prevalence has been reported in male dogs which have been castrated as well as spayed female dogs. The disease can affect any breed of dog but is most common in Boxers, Dachshund, Golden Retrievers, Irish Setters, Poodles, Miniature Schnauzers, and Greyhounds.
The factors which can trigger the development of Hypothyroidism are grouped into primary and secondary causes.
Hypothyroidism due to primary causes is the most common and is often attributed to the destruction of the Thyroid gland by inflammation, infiltration with tumor, or degeneration. Inflammatory conditions affecting the Thyroid Gland has often been blamed on the immune system.
Secondary Hypothyroidism often develops as a consequence of the existence of other conditions that can affect the production of Thyroxine by the Thyroid Glands. Certain medications such as sulfa-containing antibiotics, surgical excision of the Thyroid Glands, and radiation therapy are just some of the conditions which have been recognized to cause secondary Hypothyroidism.
When the immune system fails to recognize the Thyroid Gland and attacks the gland, a condition known as Lymphocytic Thyroiditis often arises. This type of hypothyroidism has often been observed in Dalmatians, Boxers, English Setters, and Brittany Spaniels. However, other purebred dogs such as Beagles can also suffer from Hypothyroidism.
The presence of Thyroid Tumors can also cause Hypothyroidism. However, these cases are highly uncommon unless the tumor has destroyed both lobes of the Thyroid Gland.
Idiopathic Follicular Atrophy is associated with the degeneration of the Thyroid gland. Its cause is unknown however some speculate that it is a common occurrence during the final stages of Thyroiditis.
Since most commercial dog foods from reputable feed companies already contain sufficient levels of iodine, Hypothyroidism which is linked to dietary iodine deficiency is often rare.
Dwarfism or Cretinism is a rare condition in dogs which is attributed to two endocrine abnormalities--the failure of the Hypothalamus to produce sufficient levels of thyroid-releasing hormone OR the failure of the pituitary gland to produce enough Thyroid-Stimulating hormone (TSH).