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Feline Hyperthyroidism: Spectrum of Clinical Presentations and Response to Carbimazole Therapy
Sm Anim Clin Endocrinol 11[1]:6 Jan-Apr'01 Clinical Study 0 Refs

C.B. Chastain, DVM, MS; Dave Panciera, DVM, MS; Carrie Waters, DVM, PhD
Aust Vet J 2000;78:462-465; D.G. Bucknell

BACKGROUND: Medical management of hyperthyroidism is recommended in virtually all cases, even when surgery or radioiodine treatment is planned. Renal function should be evaluated after euthyroidism is established in order to ensure that renal failure is not present. In addition, medical treatment will reduce anesthetic risk prior to surgery. Carbimazole is a thionamide antithyroid drug that undergoes rapid conversion to methimazole in cats. It seems to be associated with fewer adverse effects than methimazole, perhaps because 10 mg of carbimazole is converted to about 6 mg methimazole.

SUMMARY: Twenty-five cats with hyperthyroidism based on elevated total T4 concentration underwent evaluation including history, physical examination, complete blood count, and serum biochemical analysis. Response to carbimazole treatment, 5 mg, per os, three times per day, was monitored for 13 weeks in 14 cats. Historical signs consisted of polyphagia (56%). weight loss (52%), polyuria/polydipsia (48%), anxiety (28%), inappetence (28%), lethargy (20%), vomiting (16%), diarrhea (8%), and ataxia (8%). Hyperthyroidism was diagnosed as part of a routine screening in four cats. The most common physical examination abnormalities included palpable, enlarged thyroid gland (60%), poor body condition (54%), tachycardia (33%), heart murmur (25%), and a dry, unkempt hair coat (8%). Lymphopenia, poikilocytosis, hemoconcentration, and neutrophilia were the most common hematologic abnormalities. Serum chemistry abnormalities included elevated alanine aminotransferase (59%), azotemia (50%), hyperglobulinemia (23%), hypernatremia (18%), hyperkalemia (18 %), hyperglycemia (14%), and elevated serum alkaline phosphatase (14%). Serum T4 concentration was only mildly increased (less than 1.5 times normal) in 50% of the cases. The serum T4 concentration decreased into or below the normal range in 11 of the 14 treated cats six and 13 weeks after initiating carbimazole treatment. Clinical signs resolved in most cats. Thyroid masses were no longer palpable in any cat after six weeks of treatment. Adverse effects were reported in 10% of cats, but details were not provided. One cat developed pruritus and facial excoriations that responded to topical glucocorticoid treatment without a change in carbimazole treatment. In three cats with renal azotemia, a combination of unspecified treatment for chronic renal failure and reduction in carbimazole dose resulted in a positive response. The author concluded that cats with fewer clinical abnormalities are being diagnosed with hyperthyroidism due to earlier detection and that carbimazole is an effective and safe treatment.

CLINCAL IMPACT: Although the number of cases treated was small, carbimazole treatment in this study was associated with fewer adverse effects than with reports methimazole treatment. The intrathyroidal concentration of methimazole remains high for up to 20 hours after treatment in hyperthyroid people, so treatment with carbimazole once or twice per day rather than three times should result in a similar efficacy. Whether this would alter the incidence of adverse effects is unknown. Three cats remained hyperthyroid after 13 weeks of treatment. The dose of carbimazole administered to these cats was not stated, and it is not clear if these were cats with concurrent renal failure in which the carbimazole dose was reduced. The high incidence of azotemia points out the importance of using an antithyroid drug prior to a more permanent form of treatment such as surgical thyroidectomy or radioiodine treatment. Treatment with thyroid synthesis inhibitors such as carbimazole should not result in substantial reduction in thyroid gland mass, so it is not evident why the thyroid glands were no longer palpable after treatment.

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