CHAPTER 28 HYPOTHYROIDISM 417
Primary hypothyroidism is a defect in thyroid tissue causing a decrease in the secre-
tion of thyroid hormone.
Two forms of primary canine hypothyroidism: (naturally occurring and acquired):
Naturally occurring/congenital
Rare congenital thyroid agenesis
Dysgenesis
Dyshormogenesis (typically results in early death)
Acquired:
More than 90% of cases
Thyroid tissue is replaced or destroyed
Lymphocytic thyroiditis – most common form of canine hypothyroidism
Immune-mediated destruction of the thyroid gland
Idiopathic thyroid atrophy considered an end stage of lymphocytic thyroidi-
tis rather than a distinct disorder
Autoantibodies to thyroid antigens (antithyroglobulin antibodies).
Naturally occurring secondary canine hypothyroidism: failure of the pituitary gland
to secrete TSH; often associated with pituitary dwarfism and pituitary neoplasia
(Figures 28.1, 28.2)
Acquired secondary canine hypothyroidism (impaired TSH secretion): caused by
pituitary tumors, medications (glucocorticoids, sulfonamides, phenobarbital) or con-
current illness, including naturally occurring hypercortisolism
Hypothyroidism due to dietary iodine deficiency is rare
Alternative proposed causes include impaired conversion to active T 3 and/or
increased conversion to rT 3 due to steroid hormone abnormalities (corticosteroid or
estrogen) or poor absorption of replacement supplementation
Hypothyroidism in cats most often follows bilateral thyroidectomy or radioactive
iodine therapy for thyroid neoplasia.
SIGNALMENT/HISTORY
Most common endocrinopathy of dogs: prevalence 1:250.
Primary congenital reported in toy fox terrier, giant schnauzer, bullmastiff, Scottish
deerhound, German shepherd, Abyssinian cat.
Heritable: beagle, borzoi, Great Dane, doberman pinschers, German short-haired
pointer.
Predisposed breeds: mostly medium and large breed dogs; golden retriever, Great
Dane, doberman pinscher, Irish setter, Airedale terrier, Old English sheepdog,
dachshund, miniature schnauzer, Brittany and cocker spaniel, shar-pei, chow chow,
poodle, Irish wolfhound, English bulldog, Newfoundland, malamute, and boxer.
Peak age of onset at 5 years (mean 7 years); younger age of onset seen in larger breeds.
No sex predilection; neutered male and female dogs have an increased risk over intact
dogs.