Veterinary Information:
Hypothyroidism in Dogs

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Rare causes of hypothyroidism in dogs:
-Iodine deficiency
-Dyshormonogenesis
-Bilateral thyroid gland carcinoma (destructive tumour in the thyroid gland)
-Congenital aplasia of the thyroid gland: Hypothyroid Cretin

Common Causes:
-Acquired Primary Hypothyroidism (approx. 95%)
-Secondary Hypothyroidism (<5%)

Acquired Primary Hyperthyroidism: This is the most common cause of canine hypothyroidism. It accounts for approximately 95% of the cases. It is caused by either an immune-mediated lymphocytic thyroiditis or an idiopathic atrophy of the thyroid gland. Around 50% of the cases of acquired primary hypothyroidism are caused by a lymphocytic thyroiditis. In later stadia of the thyroiditis the parenchyma is replaced by fibrous connective tissue. Anti-thyroglobulin antibodies can be found in approximately 50% of the hypothyroid dogs. A less common cause is the destruction of the thyroid gland by a bilateral thyroid neoplasia or invasion of metastases. At least 75% of the thyroid parenchyma needs to be destroyed before symptoms of canine hypothyroidism are apparent.

Congenital Canine Hypothyroidism (Cretinism): Cretinism due to congenital hypothyroidism is a rare condition in dogs. Low T4 blood levels exist due to a deficiency of TSH or TRH: Central hypothyroidism. T4 secretion increases after TSH administration.

Predisposition:
Acquired Primary Hypothyroidism:
Mainly large dog breeds ranging from 2-6 years of age (mean age 7 years).
Heritable: Borzoi, Beagle.
High prevalence: Golden Retrievers, Irish Setters, Great Danes, Doberman pinschers, Old English Sheepdogs, Miniature Schnauzers , Dachshunds, Cocker Spaniels, Airedale Terriers.
There is an increased risk for spayed females and castrated males compared to intact dogs.
Congenital canine hypothyroidism (cretinism):
Has been reported in the Giant Schnauzer and Boxer.

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