Page 110 - Clinical Small Animal Internal Medicine

78 Section 2 Endocrine Disease

After administration, radioactive iodine is actively guaranteed. Results of a recent prospective study con­
VetBooks.ir concentrated by the thyroid gland and has a half‐life of ducted in New Zealand showed that once‐daily treat­
ment for 12 weeks with transdermal methimazole in a
eight days. It emits both beta‐particles and gamma‐radi­
ation; the beta‐particles are responsible for the majority
carbimazole administered orally.
of tissue destruction, but are only locally destructive, novel lipophilic vehicle was as effective as twice‐daily
traveling a maximum of 2 mm. Therefore, no significant While many cats have been successfully managed
damage to adjacent parathyroid tissue, atrophic thyroid long term with antithyroid drugs, it is important to
tissue, or other cervical structures is expected. The main monitor for potential side‐effects that have been associ­
limitation to widespread use of radioactive iodine is the ated with their use. In the study with the largest number
requirement for special licensing and the isolation of the of cats, 18% had side‐effects associated with methima­
cat for variable periods after treatment. This can range zole; a more recent study revealed that 44% of 39 cats
from several days to several weeks depending on state or had side‐effects. In 44 cats receiving carbimazole for
local radiation regulations and the dose administered. one year, 44% had associated side‐effects, with gastroin­
The goal of treatment is to restore euthyroidism testinal signs (decreased appetite, vomiting, diarrhea)
with the smallest possible single dose of radioactive being most common. In another study, 13% of 39 cats
iodine, while avoiding development of hypothyroid­ treated with carbimazole experienced side‐effects. It is
ism. Controversy exists as to the best method of calcu­ difficult to determine what percentage of side‐effects
lating the optimum dose for individual cats. Based on are caused by the drug versus something else such as
the majority of reported cases, posttreatment hypothy­ concurrent disease.
roidism is transient and generally uncommon (2–7% of Most adverse reactions occur within the first few
cases); even fewer cats have clinical signs or appear to weeks to months after beginning therapy and include
require thyroid hormone replacement. However, up to depression, inappetence, vomiting, and self‐induced
30% (50 of 165 cats) were hypothyroid three months excoriations of the head and neck (facial pruritus).
after radioactive iodine therapy in one study; of these, Gastrointestinal signs are less common with transder­
56% (19 of 34 hypothyroid cats with available informa­ mal administration of methimazole. Mild to serious
tion) had clinical signs of hypothyroidism and 52% (23 hematologic complications, including agranulocytosis
of 44 cats) were given thyroid hormone supplementa­ and thrombocytopenia either alone or concurrently,
tion. Thyroid hormone replacement may be needed in and more rarely immune‐mediated hemolytic anemia
some cats, especially those with concurrent kidney dis­ may also occur. Hepatic toxicity with marked increases
ease, since hypothyroidism has been associated with in bilirubin concentration and hepatic enzyme activi­
azotemia and decreased survival time in previously ties has been described in less than 2% of cats treated
hyperthyroid cats. Owners should be advised of this with methimazole. Cessation of therapy is required if
possibility, particularly if their motivation is to avoid either serious hematologic or hepatic reactions develop.
long‐term oral medication. Serum antinuclear antibodies develop in approximately
50% of cats treated with methimazole for longer than six
months, usually in cats on high‐dose therapy (>15 mg/
Antithyroid Drugs
day). Although clinical signs of a lupus‐like syndrome
Antithyroid drugs (e.g., methimazole, carbimazole) are have not been reported, decreasing the daily dosage is
commonly used for treatment of hyperthyroidism in recommended.
cats. If administered appropriately, they reliably inhibit
the synthesis of thyroid hormones and thereby lower Nutritional Management
serum thyroid hormone concentrations. These drugs
do not affect the thyroid gland’s ability to trap inorganic Production of thyroid hormone requires uptake by the
iodide or release preformed hormones. They are widely thyroid gland of sufficient amounts of iodine, which is
recommended to stabilize hyperthyroid cats prior to provided by dietary intake. The only function for ingested
surgery and are the only drugs that can be used chroni­ iodine is for thyroid hormone synthesis. This observa­
cally for management of hyperthyroidism. Almost all tion led to the hypothesis that limiting dietary iodine
cats are potential candidates unless thyroid carcinoma intake could be used to control thyroid hormone pro­
is suspected. duction and potentially manage hyperthyroidism in cats.
Antithyroid drugs used most often in cats include After more than a decade of research and development,
methimazole and carbimazole; both can be given orally a limited‐iodine therapeutic food (Hill’s® Prescription
or formulated for transdermal application. Custom for­ Diet® y/d TM Feline) containing <0.3 ppm (mg/kg) iodine
mulation of transdermal products may increase the on a dry matter basis (DMB) is now available as an option
expense of therapy and stability of the product is not for managing cats with hyperthyroidism.