Page 801 - Small Animal Internal Medicine, 6th Edition

CHAPTER 48 Disorders of the Thyroid Gland 773

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A B

FIG 48.4
(A) and (B) Eight-month-old female Giant Schnauzer litter mates. The dog on the left is
normal, whereas the smaller dog on the right has congenital hypothyroidism (cretinism).
Note the small stature; disproportionate body size; large, broad head; wide, square trunk;
and short limbs in the cretin. (From Feldman EC, Nelson RW: Canine and feline
endocrinology and reproduction, ed 4, St Louis, 2015, WB Saunders.)

at birth but become apparent between 1 and 3 months of age. Immunosuppressive drug therapy is not indicated for dogs
Dogs with cretinism have a disproportionate body size, with with these syndromes because the adverse effects of immu-
a large, broad head; a thick, protruding tongue; a wide, nosuppressive therapy and the difficulty posed by suppres-
square trunk; and short limbs (Fig. 48.4). This contrasts with sion of the immune destruction of affected endocrine glands
the proportionate dwarfism caused by growth hormone defi- outweigh the potential benefits of such therapy.
ciency. Cretins are mentally dull and lethargic, and do not
show the typical playfulness seen in normal puppies. Persis- Clinical Pathology
tence of the puppy haircoat, alopecia, inappetence, delayed The most consistent clinicopathologic findings in dogs with
dental eruption, and goiter are additional signs. Differential hypothyroidism are hypercholesterolemia and hypertriglyc-
diagnoses for failure to grow include endocrine (e.g., dwarf- eridemia; the latter is identified as lipemia. Hypercholester-
ism) and nonendocrine causes (see Box 46.5 and Fig. 46.9). olemia is identified in approximately 75% of hypothyroid
The presence of goiter is variable and dependent on the dogs, and the cholesterol concentration can exceed 1000 mg/
underlying cause. dL. Although fasting hypercholesterolemia and hypertriglyc-
eridemia can be associated with several other disorders (see
AUTOIMMUNE POLYENDOCRINE Chapter 52), their presence in a dog with appropriate clinical
SYNDROMES signs is strong evidence for hypothyroidism.
Because autoimmune mechanisms play an important role A mild normocytic, normochromic, nonregenerative
in the pathogenesis of lymphocytic thyroiditis, it is not anemia (packed cell volume [PCV] of 28%-35%) is a less
surprising that lymphocytic thyroiditis may occur in con- consistent finding, identified in approximately 30% of dogs.
junction with other immune-mediated endocrinopathies. Evaluation of red blood cell morphology may reveal an
Presumably, the immune-mediated attack is directed against increase in the numbers of leptocytes (target cells), which
antigens shared by the endocrine system. In human beings develop as a result of increased erythrocyte membrane cho-
autoimmune polyglandular syndrome type II (Schmidt’s lesterol loading. The white blood cell count is typically
syndrome) is the most common of the immunoendo- normal, and platelet counts are normal to increased.
crinopathy syndromes, and it usually consists of primary A mild to moderate increase in lactate dehydrogenase,
adrenal insufficiency, autoimmune thyroid disease, and aspartate aminotransferase, alanine transaminase, alkaline
type 1 diabetes mellitus. Autoimmune polyendocrine syn- phosphatase, and, rarely, creatine kinase activities may be
dromes are uncommon in dogs and should be suspected identified but is an extremely inconsistent finding and may
in a dog found to have multiple endocrine gland failure. not be directly related to the hypothyroid state. Mild hyper-
Hypothyroidism, hypoadrenocorticism, and, to a lesser calcemia may be noted in some dogs with congenital hypo-
extent, diabetes mellitus, hypoparathyroidism, and lympho- thyroidism. Results of urinalysis are usually normal. Polyuria,
cytic orchitis are recognized combined syndromes. In most hyposthenuria, and urinary tract infection are not typical of
affected dogs each endocrinopathy is manifested separately, hypothyroidism.
with additional disorders ensuing one by one after variable
periods (months to years). Diagnostic tests and treatment are DERMATOHISTOPATHOLOGIC FINDINGS
directed at each disorder as it is recognized, because it is not Skin biopsies are often performed in dogs with suspected
possible to reliably predict or prevent any of these problems. endocrine alopecia, especially if screening diagnostic tests