Page 825 - Small Animal Internal Medicine, 6th Edition

CHAPTER 48 Disorders of the Thyroid Gland 797

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4 weeks once control of serum T 4 concentration is achieved. thrombocytopenia (platelet counts < 75,000/mm ), leukope-
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Most cats respond to 5 to 7.5 mg of methimazole per day, nia (total white blood cell counts < 2000/mm ), and immune-
VetBooks.ir and the drug is most effective when given twice a day. mediated hemolytic anemia. Apparent hepatic toxicity or
injury occurs in less than 2% of cats receiving methimazole
Attempts at decreasing the daily dosage, the frequency of
administration, or both can take place once clinical signs
have resolved and a euthyroid state is attained, especially for and is characterized by clinical signs of liver disease (i.e.,
lethargy, anorexia, vomiting), icterus, and increased serum
cats receiving long-term methimazole treatment. alanine transaminase and alkaline phosphatase activities.
Rarely, cats are encountered that seem particularly resis- Some cats test positive for antinuclear antibodies, but the
tant to methimazole, requiring as much as 20 mg/day. The importance of this finding is not known. Development of
most common causes for apparent resistance to methimazole myasthenia gravis has also been reported with methimazole
are the inability of some clients to administer the drug to treatment. If any of these serious complications are noted,
their cats and the presence of thyroid carcinoma as the etiol- methimazole treatment should be discontinued and sup-
ogy. One alternative for inability to administer methimazole portive care given. Adverse reactions typically resolve within
is to have a compounding pharmacy incorporate methima- 1 week after methimazole treatment is discontinued. It is
zole into tasty kitty treats. Another alternative is the topical common for these potentially life-threatening adverse reac-
application of methimazole to the nonhaired pinna of the tions to recur, regardless of the dose or type of antithyroid
ear. Compounding veterinary pharmacies offer transdermal drug used; thus alternative therapy (e.g., surgery, radioactive
methimazole in a pluronic lecithin organogel (PLO) formu- iodine, iodine-restricted diet) is recommended.
lation. PLO is a permeation enhancer that disrupts the Carbimazole (Neo-Mercazole) is an antithyroid drug that
stratum corneum and allows absorption into the skin. There is converted to methimazole in vivo; it is an effective alterna-
is a slightly slower onset of control of the hyperthyroidism tive treatment if methimazole is not available. The dosage
and slightly lower efficacy, compared with oral methimazole, and frequency of administration are the same as those in oral
and some cats develop erythema and inflammation of the methimazole treatment. Adverse reactions are similar to
pinna, necessitating switching to oral methimazole. Creams those seen in cats receiving methimazole, but they occur less
can be made with methimazole at any concentration and are frequently. Cats treated with carbimazole should be moni-
usually provided in tuberculin syringes that allow the client tored in the same manner suggested for cats receiving
to place the appropriate dose on the fingertip and rub the methimazole.
cream into the pinna of the inner ear. The client must wear Long-term, twice-daily schedules for methimazole are
gloves to avoid absorption of methimazole, should alternate effective in controlling hyperthyroidism. However, feline
ears, and should wipe away any residual cream 30 to 60 hyperthyroidism is a progressive disease which cannot be
minutes after each administration. The dosage and frequency arrested with methimazole or an iodine restricted diet. Only
of administration are the same as discussed with oral radioactive iodine or thyroidectomy remove or destroy the
methimazole treatment. The bioavailability of transdermal hyperfunctioning tissue. In a recent study evaluating cats
methimazole is more variable, the overall effectiveness is not treated with methimazole for 1 to 6 years, the prevalence of
as good, and the prevalence of gastrointestinal adverse effects multifocal disease (3 or more masses), large and huge masses,
but not other adverse effects is lower, compared with oral intrathoracic thyroid tissue, and suspected thyroid carci-
methimazole. Higher doses may be required after prolonged noma increased with increasing disease duration (Peterson
treatment. One important concern in using transdermal et al., 2016). Median thyroid mass volume and serum T 4
methimazole is the lack of regulation of compounding phar- concentrations also increased with increasing disease dura-
macies; consistency between products created can vary tion; a finding which supports periodic evaluation of serum
considerably. T 4 concentrations in cats on long-term methimazole
Adverse reactions to methimazole typically occur within treatment.
the first 4 to 8 weeks of therapy (see Table 48.7). The cat
should be examined every 2 weeks during the initial 3 Iodine-Restricted Diet
months of methimazole treatment and a CBC, platelet count, A commercially available iodine restricted diet (Prescription
assessment of kidney function, and serum T 4 concentration Diet, y/d, Hill’s Pet Products) provides another option for the
evaluated at each visit. After the initial 3 months of therapy, treatment of hyperthyroidism in cats. Feeding an iodine-
CBC, platelet count, serum biochemistry panel, and serum restricted diet limits production of thyroid hormone by the
T 4 concentration should be evaluated every 3 to 6 months. thyroid gland, thereby decreasing serum T 4 concentrations.
When the dosing protocol described previously is used, leth- Clinical trials have documented a reduction in serum T 4
argy, vomiting, and anorexia occur in less than 10% of cats; concentrations when hyperthyroid cats are fed only Pre-
these mild adverse reactions are usually transient and often scription Diet y/d and do not have access to any other source
resolve despite continued administration of the drug. Mild of dietary iodine. Serum T 4 concentrations returned to the
methimazole-induced hematologic changes are seen in less reference range within 4 to 8 to weeks of initiation of the diet
than 10% of cats and include eosinophilia, lymphocytosis, and remained there for longer than a year of feeding the diet.
and transient leukopenia. More worrisome but less common It is critical that the hyperthyroid cat does not have access to
(less than 5% of cats) alterations include facial excoriations, any other source of iodine. Iodine is present in a wide array