Page 602 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

580 PART IV Specific Malignancies in the Small Animal Patient

renal disease, which is perhaps not surprising in this population of primary hyperparathyroidism. Hypercalcemia is the result of direct
older cats. The MST was 4 years in another study of cats treated effects of PTH on bone and the kidneys and indirect effects on
I compared to 2 years for cats treated with methima-
131
the intestine, mediated by vitamin D. Approximately 90% of dogs
with
VetBooks.ir zole. 322 Relatively few publications address the management of and cats with primary hyperparathyroidism have a single para-
Adenomas are most commonly diagnosed;
cats with hyperthyroidism caused by malignant thyroid disease.
thyroid mass.
335–341
Cats with thyroid carcinomas usually have larger tumor burdens, cystadenoma, carcinoma, and hyperplasia are diagnosed less fre-
and malignant cells trap and retain iodine less efficiently. 250,318 quently; and metastatic disease is extremely rare. 335–337,339–347 Two
These cats therefore are treated with higher ablative doses of 131 I, or more parathyroid masses may be found in some canine and
in the range of 20 to 30 mCi. 249,250 Another approach to the feline patients, and they may not necessarily all be of the same
treatment of thyroid carcinoma is to combine surgery and 131 I histologic type. The presence of four hyperplastic parathyroid
therapy. 239,247,250 Thyroid cysts occasionally are detected in both masses should prompt careful evaluation for causes of secondary
euthyroid and hyperthyroid cats and may be associated with either hyperparathyroidism.
benign or malignant disease. Thyroid cysts may persist after 131 I Primary hyperparathyroidism is most common in older dogs
therapy in hyperthyroid cats, therefore a surgical approach also and cats, with reported mean ages of approximately 11 years in
may be needed in these cases. 251 dogs 337,339 and 13 years in cats. 335 A breed predisposition has been
Ultrasound-guided percutaneous ethanol injection has been reported in keeshond dogs, in which the disease appears to fol-
evaluated as a treatment for feline hyperthyroidism. Cats with low an autosomal dominant mode of inheritance, although the
solitary adenomas have a good response, with resolution of clini- affected gene has not yet been identified in this breed. 337,341,343,348
cal signs persisting for longer than 12 months. 323 This technique is It is not clear whether a breed predilection exists in cats.
not recommended for bilateral hyperplasia. 324 Ultrasound-guided The clinical signs of hyperparathyroidism result from hyper-
percutaneous radiofrequency heat ablation has been shown to be calcemia; they include polyuria/polydipsia, weakness, lethargy,
ineffective for long-term control of hyperthyroidism. 325 Given the decreased appetite, weight loss, muscle wasting, vomiting, and
ready availability of permanent effective treatments for unilateral trembling. It is not uncommon for owners to detect no clinical
or bilateral disease, these alternative treatments are unlikely to be signs in affected dogs or cats, and the hypercalcemia is diagnosed
used widely. when blood is drawn for a routine health check or for investiga-
Chronic kidney disease (CKD) is a relatively common prob- tion of an unrelated problem. However, signs can be subtle and
lem in older cats, and concurrent CKD and hyperthyroidism may be recognized only in retrospect, after the hyperparathyroid-
frequently occur in this population. 326 The hyperthyroid state ism has been treated and the hypercalcemia has resolved. In a large
increases the glomerular filtration rate (GFR), 327,328 thereby case series, the most common clinical problems reported in dogs
reducing serum creatinine values. The implications of this are with hyperparathyroidism were related to the lower urinary tract,
that hyperthyroid cats with normal serum creatinine values actu- usually associated with urolithiasis or urinary tract infection. 337
ally may have concurrent masked CKD and that decline in renal Specific physical examination abnormalities are rare in dogs and
function is a risk of all effective treatments for feline hyperthy- cats. A palpable parathyroid mass has been reported in some cats
roidism, with some nonazotemic cats becoming azotemic, or the with hyperparathyroidism, but this is an extremely rare finding in
potential for worsening of preexisting azotemia. 319,327,329,330 This dogs. 336,337,347
decline in renal function occurs within 1 month after treatment Hyperparathyroidism usually is diagnosed after hypercal-
and appears to remain stable thereafter. 319,331 Measurement of the cemia is found on a serum biochemistry profile, either as an
pretreatment GFR may help predict which cats will become azo- incidental finding or when a problem such as calcium oxalate
temic after resolution of the hyperthyroidism 319,330 ; however, this urolithiasis, polyuria/polydipsia, or weakness is investigated.
is impractical for most patients. A recent study found that a high The presence of hypercalcemia should be verified by measur-
serum symmetric dimethylarginine (SDMA) concentration in a ing the serum ionized calcium, with appropriate careful sample
hyperthyroid cat can help predict the development of azotemia handling. 349,350 Hypercalcemia has many causes in dogs and
after treatment, but this has poor sensitivity. 332 Unfortunately, no cats, 351,352 and diagnostic tests may be performed to investigate
readily available clinical data can predict the effects of therapy on several possible causes simultaneously. (The reader is directed to
renal function in an individual cat. 326,333 For this reason, many Chapter 5 for a further discussion of the causes of hypercalcemia
clinicians recommend a therapeutic trial with methimazole before in dogs and cats.) Hypercalcemia resulting from primary hyper-
definitive therapy for feline hyperthyroidism. 333 This may have parathyroidism often is accompanied by hypophosphatemia, or
value in providing owners with information about the likely con- a serum inorganic phosphorus level at the low end of the refer-
sequence of therapy for these cats, but regardless of the detected ence range. This finding is not pathognomonic for hyperpara-
change in renal function, effective therapy for hyperthyroidism thyroidism and can be associated with humoral hypercalcemia of
still is required in these patients. One study showed that the malignancy, but it can assist in ranking the differential diagno-
development of azotemia was not significantly associated with ses because vitamin D toxicosis and renal failure both would be
the survival of cats treated for hyperthyroidism, 334 but the same expected to cause hyperphosphatemia. The diagnosis of hyper-
group also demonstrated a significantly shorter ST in cats with parathyroidism is confirmed by documenting an inappropriately
iatrogenic hypothyroidism that became azotemic after treatment high serum PTH level in the presence of ionized hypercalcemia.
compared with those that remained nonazotemic. 321 It is important to note that PTH frequently is within the refer-
ence range in patients with hyperparathyroidism, with 73% of
Parathyroid Tumors cases reported to have a normal PTH in one large series. 337 A
normal PTH in the face of hypercalcemia is an abnormal finding
Parathyroid tumors are uncommon in dogs and rare in cats. These because PTH should be suppressed as serum calcium increases.
tumors arise from the chief cells and autonomously secrete para- The lack of suppression of PTH indicates loss of the normal neg-
thyroid hormone (PTH), leading to hypercalcemia as a result of ative feedback effects of calcium due to autonomous hormone