Canine Hypothyroidism | Hypothyroidism in Dogs

Canine Hypothyroidism | Hypothyroidism in Dogs
Structural or functional abnormalities of the thyroid gland can lead to deficient production of thyroid hormones. A convenient classification scheme for hypothyroidism in dogs has been devised that is based on the location of the problem within the hypothalamic-pituitary-thyroid gland complex. Primary hypothyroidism in dogs is the most common form of this disorder; it results from problems within the thyroid gland, usually destruction of the thyroid gland.

The two most common histologic findings in this disorder are lymphocytic thyroiditis and idiopathic atrophy of the thyroid gland. Lymphocytic thyroiditis is an immune-mediated disorder characterized by a diffuse infiltration of lymphocytes, plasma cells, and macrophages into the thyroid gland. The factors that trigger the development of lymphocytic thyroiditis are poorly understood, but genetics factors undoubtedly play a role.

Idiopathic atrophy of the thyroid gland is characterized by loss of the thyroid parenchyma. There is no inflammatory infiltrate, even in areas where small follicles or follicular remnants are present. The cause of idiopathic thyroid atrophy is not known, but it may be a primary degenerative disorder. It may also represent an end stage of autoimmune lymphocytic thyroiditis.

Hypothyroidism symptoms in dogs

Clinical signs of the more common forms of primary hypothyroidism in dogs usually develop during middle age. Clinical signs tend to develop at an earlier age in breeds at increased risk than other breeds. There is no apparent sex-related predilection.

Dog Breeds that have an increased prevalence of hypothyroidism:

English Pointer
German Wirehaired Pointer
American Staffordshire Terrier
American Pit Bull Terrier
Giant Schnauzer
Golden Retriever
Chesapeake Bay retriever
Brittany Spaniel
Australian Shepherd
English Setter
Skye Terrier
Old English Sheepdog
Petit Basset Griffon
Rhodesian Ridgeback
Shetland Sheepdog
Siberian Husky
Doberman Pinscher
Cocker Spaniel

Clinical signs are quite variable and depend in part on the age of the dog at the time a deficiency in thyroid hormone develops. Clinical signs may also differ between breeds. For example, truncal alopecia may dominate in some breeds, whereas thinning of the haircoat dominates in other breeds. In adult dogs, the most consistent clinical signs of hypothyroidism result from decreased cellular metabolism and its effect on the dog's mental status and activity.

Most dogs with hypothyroidism show more mental dullness, lethargy, exercise intolerance or unwillingness to exercise, and a propensity to gain weight without a corresponding increase in appetite or food intake. These signs are often gradual in onset, subtle, and not recognized by the owner until after thyroid hormone supplementation has been initiated. Additional clinical signs of hypothyroidism in dogs typically involve the skin, less commonly, the neuromuscular system.