Page 600 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

578 PART IV Specific Malignancies in the Small Animal Patient

heart murmur, gallop rhythm, premature beats, and poor hair hyperthyroidism. 244,245,289 Unilateral uptake occurs in cats with a
coat. However, because measurement of serum thyroxine (T ) is solitary adenoma and atrophy of the normal contralateral gland.
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Bilateral uptake, even if asymmetric, is indicative of adenoma-
a common component of wellness laboratory testing in cats and
VetBooks.ir because awareness of the condition is high, this “typical” presenta- tous hyperplasia. Thyroid scintigraphy is particularly useful for
revealing the presence of ectopic thyroid tissue or multiple areas
tion of feline hyperthyroidism likely is becoming less common,
with an increasing proportion of cats being diagnosed with mild of hyperfunctioning thyroid tissue. Metastatic disease caused by
signs, no signs, or atypical signs. 239,272 In a recent study of more thyroid carcinoma may be detected by scintigraphy, and the pat-
than 400 hyperthyroid cats, most had lost body weight, but this tern of uptake of radionuclide may be suggestive of the presence
was associated with muscle wasting, and most cats had an ideal or of malignant disease 245,289 ; however, other studies have demon-
overweight body condition score at diagnosis, with only approxi- strated that no scintigraphic findings can distinguish definitively
mately one third determined to be underweight. 273 No histori- between benign and malignant thyroid disease in all hyperthyroid
cal or physical examination findings can distinguish definitively cats. 239,244,249
between benign and malignant thyroid disease in cats. Treatment options for feline hyperthyroidism include anti-
The serum total T concentration is highly sensitive and spe- thyroid drugs, dietary management, surgical thyroidectomy,
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cific for the diagnosis of feline hyperthyroidism, 239,274 but false and radioactive iodine therapy. 239,253 Homeopathic therapy is
positive or false negative results are possible. 275 Approximately ineffective. 290 Methimazole is the most widely used antithyroid
10% of hyperthyroid cats have a total serum T concentration drug in North America. 291 Carbimazole is used more widely in
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within the reference range. 275 This may be due to the presence Europe. 253,292 These are thioureylene drugs that inhibit thyroid
of early disease, normal fluctuations in serum thyroid hormone hormone synthesis by interfering with the oxidation of iodide,
concentrations, and/or the presence of concurrent nonthyroidal iodination of tyrosyl residues in thyroglobulin, and the coupling
illness. 274,276 Thus the clinician always must consider test results of iodotyrosines to iodothyronines. 239 Methimazole and carbima-
in light of the clinical signs in the patient. If hyperthyroidism zole are both highly effective in lowering serum thyroid hormone
is suspected in a cat with a normal total T concentration, the concentrations and controlling hyperthyroidism. Carbimazole is
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total T should be measured again in 1 to 2 weeks, particularly converted to methimazole in the body, and a dose of 5 mg of
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if the total T is in the upper half of the reference range. Free T carbimazole is considered to be equivalent to 3 mg of methima-
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measurement also may aid in the diagnosis of hyperthyroidism zole. 239 Methimazole is usually administered at a starting dose of
when the total T is within the reference range. However, this test 2.5 mg orally twice daily for 2 weeks. 240,293 Based on clinical signs
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should be used only in cats with clinical signs of hyperthyroidism and serum T levels, the dosage can be adjusted incrementally
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in which the total T is in the high normal range. Free T con- with monitoring of serum T concentrations. Once daily admin-
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centrations can be high in cats with nonthyroidal illness, 274,277,278 istration of methimazole has been reported to be less effective, 294
and these patients would be expected to have low serum total T but this approach can be successful in some cats, particularly those
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concentrations. Thus free T should never be used as a screening that need very low doses to control their disease. Carbimazole usu-
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test for hyperthyroidism. For patients in which hyperthyroidism ally is administered 2 or 3 times daily, but a controlled-release for-
is suspected but not confirmed by measurement of the total or mulation has been shown to be effective when administered once
free T , additional tests have been used to confirm the diagno- daily. 295 For cats that are difficult to medicate orally or that have
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sis. These include thyroid scintigraphy, the TSH stimulation test, GI side effects, methimazole compounded in pluronic lecithin
the thyrotropin-releasing hormone (TRH) stimulation test, and organogel (PLO) 296–298 or in a lipophilic formulation 299 can be
the triiodothyronine suppression test. Both the TSH and TRH applied topically to the ear pinna. Transdermal carbimazole also
stimulation tests are of limited utility, 279,280 and the triiodothyro- has been shown to be effective. 300 Although transdermal applica-
nine suppression test can provide useful information but relies on tion of antithyroid medications appears to be safe and effective and
significant owner and patient compliance. 281,282 Measurement of often is more convenient for cat owners, dose adjustments, partic-
the serum TSH concentration is very sensitive but poorly specific ularly dose increases, often are necessary for long-term control. 301
for the diagnosis of feline hyperthyroidism, 283 and the results of Approximately 10% to 25% of patients treated with methimazole
this assay should be interpreted in conjunction with clinical signs develop adverse effects, including lethargy, anorexia, vomiting,
concentrations. Thyroid function facial excoriations, hepatotoxicity, bleeding diatheses, and blood
and the serum total and free T 4
tests cannot be used to differentiate benign and malignant tumors. dyscrasias. 239,291,293,302 GI side effects are often self-limiting or can
The diagnostic workup for cats with hyperthyroidism mini- be avoided by transdermal drug delivery. Blood dyscrasias are rare
mally should include a baseline CBC, serum biochemistry profile, but most likely to occur within the first 3 months of treatment;
urinalysis, and blood pressure measurement. 284 These tests may therefore CBCs should be monitored most closely during this
reveal abnormalities caused by hyperthyroidism (e.g., increased time. Medication should be discontinued in patients that experi-
liver enzyme activity) and also provide evidence of concurrent ence facial excoriations, blood dyscrasias, or hepatotoxicity. Car-
disease. Additional diagnostic tests that may be recommended bimazole or transdermal methimazole are likely to have the same
include thoracic radiography, electrocardiography, and echocar- effects and therefore should not be used in patients experiencing
diography. 243,285,286 An abdominal ultrasound examination typi- these adverse effects with oral methimazole. These drugs often
cally is performed only if the clinician suspects concurrent illness are used for assessing the effect of resolution of hyperthyroidism
because this test rarely provides evidence against recommending on renal function and preparing a cat for anesthesia and thyroid-
definitive treatment for hyperthyroidism. 287 If performed, an ectomy. They also frequently are used as a long-term treatment
abdominal ultrasound examination may reveal bilateral moder- modality, but it is important to note that they have no antitumor
ate adrenomegaly in hyperthyroid cats. 288 Thyroid scintigraphy activity and no cytotoxic effect on thyroid follicular cells. A recent
(most commonly using 99m Tc-pertechnetate) is valuable for study demonstrated that the duration of disease, despite antithy-
determining the anatomic extent of functional thyroid tissue and roid drug therapy, is correlated positively with the development of
for planning therapy, in addition to confirming the diagnosis of more severe hyperthyroid disease, characterized by increasing size,