Page 601 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 26 Tumors of the Endocrine System 579

volume, and number of thyroid nodules, in addition to suspected antithyroid medications or β-adrenergic blockers should be con-
increasing prevalence of thyroid carcinoma. 246 In other words, the sidered. The most significant intraoperative complication of thy-
roidectomy in hyperthyroid cats may be cardiac dysrhythmias.
309
longer a patient is managed with these drugs, the more likely it is
VetBooks.ir to develop disease that may be less responsive to definitive therapy Otherwise, the surgery is not technically demanding. 242,309 Hypo-
calcemia resulting from transient or permanent hypoparathyroid-
with
I. Other medical therapies that have been used to treat
131
feline hyperthyroidism include ipodate and iopanoic acid 303,304 ; ism is the most commonly reported postoperative complication,
however, these are unlikely to be effective for long-term control with rates ranging from 6% to 15%. 242,309 Other potential com-
and rarely are used. plications include hypothyroidism and, in rare cases, Horner’s syn-
A more recent development in the management of feline hyper- drome or laryngeal paralysis. All surgically excised tissue should be
thyroidism is the use of an iodine-restricted diet to control the submitted for histopathology to rule out the presence of a thyroid
disease. Hill’s Prescription Diet y/d Feline is a commercially avail- carcinoma. Cats with thyroid carcinoma that undergo thyroidec-
able diet that is extremely restricted in iodine, containing only 0.2 tomy usually experience improvement in their clinical signs, but
ppm, compared to the recommended minimum of 0.46 ppm for most remain hyperthyroid or develop recurrent hyperthyroidism
adult cats. 253 Dietary management appears to be effective in most within a few months of surgery. 247,250 Cats with ectopic hyper-
cats 305 ; however, up to 25% of cats continue to have increased plastic thyroid tissue also are at risk for postoperative recurrence of
serum total T concentrations after several weeks on the diet, and hyperthyroidism. 242 Radioactive iodine therapy is recommended
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not all clinical signs of hyperthyroidism appear to resolve. 306,307 for patients with thyroid carcinoma or ectopic hyperplastic thy-
A significant disadvantage of this approach is that the cat must be roid tissue.
fed the prescription diet exclusively. Feeding of other diets (even Radioactive iodine, or 131 I therapy, generally is regarded as
in very small amounts), treats, flavored medications, or hunting the treatment of choice for cats with hyperthyroidism, particu-
negates the effects of the highly iodine-restricted diet. Therefore larly those with bilateral thyroid hyperplasia, ectopic thyroid tis-
a 131
compliance can be poor, particularly in multicat households. Fur- sue, or thyroid carcinoma. I has a half-life of 8 days and emits
thermore, as with antithyroid medications, dietary therapy does both beta and gamma radiation. Beta particles, which account for
not inhibit the growth and progression of the primary thyroid 80% of the tissue damage, travel a maximum of 2 mm in tissue
lesion in these cats, perhaps leading to an increased risk of the and have an average path length of 400 μm. They therefore cause
development of malignant disease over time. However, this is only local destruction while sparing adjacent hypoplastic thyroid tis-
a theoretical concern, because no studies have been published sue, parathyroid glands, and other cervical structures. The dose of
evaluating the long-term risks of dietary management of feline 131 I can be calculated from tracer kinetic studies, 312,313 but these
hyperthyroidism. An additional consideration is that scintigraphy are rarely performed. The administration of a fixed dose of 131 I
studies showed that cats that consume the iodine-restricted diet is reported by some authors, 314–317 whereas others use doses that
for at least 6 months had a greater percentage uptake of 123 I by take into account variables such as the number or size of thyroid
the thyroid gland 8 hours after isotope administration compared nodules, the patient’s body weight, the severity of the clinical signs,
to baseline values. 305 Further studies are needed to determine if or the magnitude of elevation in the serum total T . 239,311,318,319
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consumption of this diet affects the response to 131 I therapy in 131 I usually is administered by the subcutaneous route because it
hyperthyroid cats. Whenever possible, definitive therapy is recom- is effective, less stressful for the patient, and safer for personnel. 318
mended for cats with hyperthyroidism; however, dietary manage- For cats with benign thyroid disease, reported 131 I doses typically
ment may be a valuable option for cats with concurrent illnesses range from 2 to 6 mCi. Regardless of the wide variety of dosing
and for owners who cannot medicate their cats or are unable to strategies used, overall less than 5% of cats remain hyperthyroid
pursue definitive therapy. or experience relapse of clinical signs after 131 I therapy. When
Definitive therapy for feline hyperthyroidism currently consists treatment failure occurs, a second treatment often is curative. One
of surgical thyroidectomy or radioactive iodine. Surgical excision recent study compared the efficacy of a 2 mCi dose of 131 I to a
of the affected thyroid lobe(s) is an effective treatment, 239,242,308,309 “standard” dose of 4 mCi for cats with serum T concentrations
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although it is less commonly performed as access to radioactive in the range of 4 to 10.3 μg/dL. 316 No significant difference was
iodine therapy expands. 239 Although most cats have bilateral dis- seen in the prevalence of persistent hyperthyroidism between the
ease, this may be asymmetric and not apparent on palpation or two groups; the lower dose resulted in euthyroidism in greater
surgical exploration. Thus thyroid scintigraphy is recommended than 95% of cats by 6 months after treatment. The use of a lower
before surgery to determine whether unilateral or bilateral thy- dose is appealing because it reduces radiation exposure, shortens
roidectomy is necessary. 242 Intracapsular and extracapsular thy- quarantine times, and reduces costs. The proportion of cats that
roidectomy techniques have been described. 239,308,310 When develop persistent hypothyroidism after 131 I therapy varies among
bilateral thyroidectomy is indicated, preservation of one of the studies, and the risk of this has been suggested to be higher in cats
parathyroid glands is important to maintain calcium homeostasis. with scintigraphic evidence of bilateral disease. 320 In the recent
Extracapsular thyroidectomy is most commonly performed. 220,309 study comparing a 2 mCi dose to a 4 mCi dose, overt or subclini-
This involves removal of the thyroid gland and its capsule, using cal hypothyroidism was more likely in the group that received 4
gentle dissection and meticulous hemostasis, from caudal to cra- mCi. 316 Minimizing the risk of iatrogenic hypothyroidism after
nial. At the cranial extent of the thyroid, the external parathyroid 131 I therapy is important because cats with iatrogenic hypothy-
gland must be identified and its blood supply preserved. 220,309 The roidism that become azotemic after treatment have shorter STs
intracapsular technique involves dissection of the thyroid gland than cats that remain nonazotemic. 321 In one large study of hyper-
within its capsule in an effort to preserve the external parathyroid thyroid cats treated with 131 I, the MST was 2 years, with survival
gland. 220,309 Unfortunately, this technique can lead to recurrence rates at 1, 2, and 3 years of 89%, 72%, and 52%, respectively. 311
of hyperthyroidism as the result of remnants of thyroid tissue that The most common causes of death or euthanasia were cancer or
are left with the capsule. 220,309 Hyperthyroid cats often are poor
anesthetic candidates, and preoperative stabilization with oral a References 239, 247, 249, 250, and 311.