Page 787 - Small Animal Internal Medicine, 6th Edition

CHAPTER 47 Disorders of the Parathyroid Gland 759

TABLE 47.1
VetBooks.ir Biologic Actions of the Hormones That Affect Calcium and Phosphorus Metabolism NET EFFECT

HORMONE BONE KIDNEY INTESTINE SERUM Ca SERUM PO 4

Parathyroid hormone Increased bone resorption ↑Ca absorption No direct effect ↑ ↓
↑PO 4 excretion
Calcitonin Decreased bone resorption ↓Ca resorption No direct effect ↓ ↓
↓PO 4 resorption
Vitamin D Maintains Ca transport system ↓Ca resorption ↑Ca absorption ↑ ↑
↑PO 4 absorption

↑, Increased; ↓, decreased; Ca, calcium; PO 4 , phosphorus.

A B

FIG 47.1
(A) Surgical site in a 12-year-old dog with primary hyperparathyroidism (PHP). A
parathyroid adenoma (arrow) can be seen in the thyroid lobe. (B) Gross appearance of
parathyroid adenoma (arrow) and thyroid lobe after removal from the dog in A.

involvement of multiple parathyroid glands suggests hyper- dominant, genetically transmitted disease in this breed. The
plasia, adenoma involving two parathyroid glands and age at the time of diagnosis of PHP in cats has ranged from
hyperplasia involving only one gland have been identified in 8 to 20 years, with a mean age of 13 years. Most affected cats
dogs with PHP. In addition, hyperplasia caused by renal and have been mixed breed and Siamese. No apparent sex pre-
nutritional secondary hyperparathyroidism may not cause disposition has been reported.
uniform enlargement of the parathyroid glands even though
the stimulus for enlargement is presumably the same for CLINICAL SIGNS
each gland. Differentiating hyperplasia from adenoma has Clinical signs of PHP result from the physiologic actions of
important prognostic implications. The surgical removal of excessive PTH secretion rather than from the space-
parathyroid adenoma(s) results in a cure, if at least one occupying nature of the tumor. Clinical signs are caused by
normal parathyroid gland remains to prevent hypoparathy- hypercalcemia, which is the hallmark of this disorder, and
roidism. In contrast, hypercalcemia caused by parathyroid by the presence of cystic calculi and lower urinary tract
hyperplasia may persist or recur weeks to months after infection, which are consequences of the hypercalcemia.
surgery if the remaining grossly normal-appearing parathy- Clinical signs are absent in most dogs and cats with the
roid tissue is hyperplastic at the time of surgery or becomes mildest form of PHP, and hypercalcemia is discovered only
hyperplastic in the future. after a serum biochemistry panel is performed, often for
unrelated reasons. When clinical signs do develop, they ini-
SIGNALMENT tially tend to be nonspecific and insidious in onset. Clinical
The age at which clinical signs of PHP appear in dogs ranges signs in dogs are typically renal, gastrointestinal, and neuro-
from 4 to 16 years, with a mean age of 10 years. No sex- muscular in origin (Box 47.1). The most common clinical
related predilection has been noted. Any breed of dog and signs in cats with PHP are lethargy, anorexia, and vomiting.
mixed-breeds can be affected, although PHP is most com- Less common clinical signs in cats include constipation,
monly diagnosed in the Keeshond and is an autosomal polyuria, polydipsia, and weight loss.