Page 822 - Small Animal Internal Medicine, 6th Edition

794 PART VI Endocrine Disorders

although suppression of more than 50% below the baseline in the hope of providing a permanent cure for the disease;
value occurs in normal but not hyperthyroid cats. Serum T 3 oral antithyroid drugs and an iodine-restricted diet only
VetBooks.ir concentrations are used to determine whether the client has control the hyperthyroidism and must be given daily to
achieve and maintain their effects.
successfully administered the thyroid medication to the cat.
Serum T 3 concentration measured in the postpill blood
sample should be increased compared with results obtained Initial Treatment Recommendation
before the test is initiated in all cats properly tested, regard- Hyperthyroid cats should be treated initially with an oral
less of the status of thyroid gland function. antithyroid drug (i.e., methimazole, carbimazole) to reverse
hyperthyroid-induced metabolic and cardiac derangements,
Thyroid Scintigraphy decrease the anesthetic risk associated with thyroidectomy,
Thyroid scintigraphy identifies functional thyroid tissue and and assess the impact of treatment on kidney function.
is used as a diagnostic test in cats with suspected occult Feeding an iodine-restricted diet (Prescription Diet y/d,
hyperthyroidism; to identify ectopic thyroid tissue in cats Hill’s Pet Products) should be considered in situations where
with appropriate signs of hyperthyroidism and increased owners are unable to administer methimazole to their cat.
serum T 4 concentrations but no palpable thyroid nodule in Hyperthyroidism may mask CKD in some cats (see p. 790);
the neck; to identify sites of metastasis in cats with thyroid azotemia may develop or worsen and clinical signs of CKD
carcinoma; and to provide guidance for developing the best may develop after treatment of the hyperthyroid state.
treatment plan, especially if thyroidectomy is being consid- Because it is not easy to determine what impact the hyper-
ered. Radioactive technetium-99m (pertechnetate) is used thyroid state is having on kidney function, it is preferable to
for routine imaging of the thyroid gland in cats. It has a short treat cats with reversible therapy (i.e., methimazole) until the
physical half-life (6 hours), is concentrated within function- impact of hyperthyroidism on kidney function can be deter-
ing thyroid follicular cells, and reflects the trapping mecha- mined. If the serum creatinine concentration remains static
nism of the gland. Because antithyroid drugs do not affect or improves after resolution of hyperthyroidism with
the trapping mechanism of the thyroid pump, thyroid scin- methimazole, a more permanent treatment can be recom-
tigraphy can be done in cats being treated with antithyroid mended. Idiopathic hypothyroidism can decrease GFR or
drugs. Salivary glands and the gastric mucosa also concen- increase serum creatinine concentration, and must be
trate pertechnetate; it is excreted by the kidneys. avoided during methimazole treatment. If the serum creati-
Scanning of the thyroid provides a picture of all function- nine concentration increases significantly (i.e., to IRIS stage
ing thyroid tissue and permits the delineation and localiza- 3 or higher) during methimazole therapy, the treatment pro-
tion of functioning as opposed to nonfunctioning areas of tocol for methimazole should be modified to attain the best
the thyroid but does not differentiate between adenomatous possible control of both disorders, and treatment for CKD
hyperplasia, adenoma, and carcinoma. Fig. 48.14 shows the should be instituted.
similarity between the size and shape of the thyroid lobes
and the similarity of radionuclide uptake by the thyroid and Oral Antithyroid Drugs
salivary glands in a normal cat. This 1 : 1 ratio of salivary Oral antithyroid drugs include methimazole, propylthioura-
gland to thyroid lobe uptake is the standard by which the cil, and carbimazole. Oral antithyroid drugs are inexpensive,
status of the thyroid should be judged. Findings in most readily available, relatively safe, and effective in the treatment
hyperthyroid cats are markedly abnormal and usually easy of hyperthyroidism in cats. They inhibit the synthesis of
to interpret (see Figs. 48.14 to 48.16). thyroid hormone by blocking the incorporation of iodine
into the tyrosyl groups in thyroglobulin and by preventing
Cervical Ultrasound the coupling of these iodotyrosyl groups to form T 3 and T 4 .
Ultrasonographic evaluation of the thyroid gland can be Antithyroid drugs do not block the release of stored thyroid
used to confirm the origin of the palpable cervical mass, hormone into the circulation and do not have antitumor
differentiate unilateral versus bilateral thyroid lobe involve- actions. In our experience, oral antithyroid drugs do not
ment, assess the size of the thyroid mass(es), and provide interfere with results of pertechnetate scans or radioactive
guidance for developing the best treatment plan (Fig. 48.20). iodine therapy; however, many treatment centers request
Ultrasound does not provide information on the functional that methimazole be withdrawn 1 to 2 weeks before pertech-
status of the thyroid mass and should not be used in estab- netate scan or radioactive iodine treatment. Indications for
lishing the diagnosis of hyperthyroidism. Rather, cervical oral antithyroid drugs include (1) test treatment to normal-
ultrasound should be used as an adjunctive tool for locating ize serum T 4 concentrations and assess the effect of resolving
cervical thyroid tissue. hyperthyroidism on kidney function; (2) initial treatment to
alleviate or eliminate any medical problems associated with
Treatment the syndrome before thyroidectomy is performed or before
Hyperthyroidism in cats can be managed by oral antithyroid the hospitalization required for radioactive iodine treatment;
medications, thyroidectomy, radioactive iodine, or an iodine- and (3) long-term treatment of hyperthyroidism.
restricted diet (Table 48.6). All four modes of therapy are Methimazole (Felimazole, Dechra Veterinary Products)
effective. Surgery and radioactive iodine treatments are used is currently the antithyroid drug of choice because the