Small Animal Dermatology, 3rd edition

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.