Page 816 - Small Animal Internal Medicine, 6th Edition

788 PART VI Endocrine Disorders

Prognosis unaffected thyroid lobe is nonfunctioning and atrophied
The prognosis for adult cats with hypothyroidism receiving because of the suppressive effects of the hyperactive thyroid
VetBooks.ir appropriate therapy is excellent. The prognosis for kittens tissue on TSH secretion. Approximately 65% of hyperthyroid
cats have involvement of both thyroid lobes (Fig. 48.15).
with congenital hypothyroidism is guarded and depends on
the severity of skeletal changes at the time treatment is initi-
enlarged in 10% to 15% and asymmetrically enlarged in the
ated. Many of the clinical signs resolve and body size may Among these cats the thyroid lobes are symmetrically
increase if hypothyroidism is identified early, before abnor- remainder. Approximately 10% of enlarged thyroid lobes
mal bone and joint development occurs. descend to the thoracic inlet and can be difficult to palpate.
Less than 5% of thyrotoxic cats have hyperactive thyroid
tissue in an ectopic location, most commonly the anterior
HYPERTHYROIDISM IN CATS mediastinum, with or without a palpable mass in the neck
(Fig. 48.16). Functional thyroid carcinoma is the most likely
Etiology diagnosis if more than two thyroid masses are present (see
Hyperthyroidism is a multisystemic disorder resulting from Fig. 48.16), although adenomatous hyperplasia of ectopic
excessive production and secretion of T 4 and T 3 by the sites in addition to the thyroid lobes is possible. Some cats
thyroid gland and is almost always a result of chronic intrin- with thyroid carcinoma initially have only one or two thyroid
sic disease in one or both thyroid lobes. One or more usually masses, emphasizing the importance of histologic evaluation
small, discrete thyroid masses are palpable in the ventral of surgically removed tissue.
region of the neck in most cats with hyperthyroidism. Fol- The pathogenesis of adenomatous hyperplastic changes
licular cell adenoma and multinodular adenomatous hyper- in the thyroid gland remains unclear. It has been postulated
plasia are the most common histologic findings. Both are that immunologic, infectious, nutritional, environmental, or
benign and both may occur together within the same thyroid genetic factors may interact to cause pathologic changes.
lobe. Less common are thyroid adenomas that cause the Nondietary risk factors identified by case-controlled studies
lobes to become markedly enlarged and distorted; thyroid include increasing age; indoor housing; use of fertilizers, her-
carcinoma accounts for less than 5% of clinical cases. bicides, flea powders, and sprays; and use of cat litter. Epide-
One or both thyroid lobes can be affected in thyrotoxic miologic studies have identified consumption of commercial
cats. Approximately 30% of hyperthyroid cats have involve- canned cat foods as a risk factor for development of hyper-
ment of a single thyroid lobe (Fig. 48.14). The contralateral thyroidism, suggesting that a goitrogenic compound may

A B
A B
FIG 48.14
(A) Sodium pertechnetate scan of the head, neck, and FIG 48.15
proximal thorax of a healthy cat. Note that the uptake of (A) Sodium pertechnetate scan of the head, neck, and
pertechnetate (i.e., darkness) is comparable between the proximal thorax of a cat with hyperthyroidism caused by
two thyroid lobes (solid arrow) and the salivary glands bilateral, asymmetric disease affecting both thyroid lobes
(broken arrow). (B) Sodium pertechnetate scan of the head, (arrows), with the right lobe more severely involved. This is
neck, and proximal thorax of a cat with hyperthyroidism the most common form of the disease. (B) Sodium
caused by unilateral disease affecting the right thyroid lobe pertechnetate scan of the head, neck, and proximal thorax
(arrow). Note the difference in uptake of pertechnetate of a cat with hyperthyroidism caused by bilateral, symmetric
between the hyperfunctioning thyroid lobe and the salivary disease affecting both thyroid lobes (arrows). Hypocalcemia
glands. after bilateral thyroidectomy is a major concern.