Feline hyperthyroidism

Weigh the pros and cons of medical, surgical management
Feb 01, 2006

Treatment options for hyperthyroid cats are short- or long-term use of orally or topically administered antithyroid drugs, radioactive iodine 131 (131I) or surgical thyroidectomy.
Hyperthyroidism can be diagnosed in most cats based on clinical signs consistent with the disease, physical examination findings including a palpable thyroid nodule and an increased serum total T4 concentration.

Routine laboratory evaluation including a complete blood count, serum chemistry profile, urinalysis and serum T4 concentration are valuable in diagnosing hyperthyroidism and help to identify concurrent diseases.

Most hyperthyroid cats show mild to moderate increases of serum alanine aminotransferase or serum alkaline phosphatase activities. Measurement of random serum T4 concentrations has been reliable in identifying cats with hyperthyroidism. Fluctuation of serum thyroid hormone concentrations and suppression of increased thyroid hormone concentrations resulting from concurrent non-thyroidal disorder may explain why some cats with suspected hyperthyroidism have a serum T4 concentration within the reference interval. Therefore, the diagnosis of hyperthyroidism should not be ruled out on the basis of one serum T4 concentration within the reference interval, especially if the cat has clinical signs and a palpable thyroid nodule.

Recommendation is to measure serum T4 concentration days to weeks after the initial result was obtained and to rule out non-thyroidal illness.

If the serum T4 concentration remains non-diagnostic and hyperthyroidism is still suspected, thyroid function should be reassessed after resolution of non-thyroidal illness, a serum-free T4 concentration may be assayed, a thyroid scan may be obtained, and lastly, a T3 suppression test or thyrotropin-releasing hormone stimulation test may be done.

Table 1: Advantages and disadvantages of the three major forms of therapy for feline hyperthyroidism*
The goal of therapy is to control or inhibit excessive secretion of thyroid hormones. Treatment options include short- or long-term use of orally or topically administered antithyroid drugs, radioactive iodine 131 (131I) or surgical thyroidectomy. Each form of therapy has advantages and disadvantages that must be considered and are summarized in Table 1.

Medical therapy is a reversible form of treatment, whereas surgery and administration of 131I are permanent. Trial transient therapy with methimazole (oral or topical) should be done prior to use of a permanent treatment method.

Methimazole inhibits several steps in thyroid hormone synthesis and is very effective in restoring euthyroidism in most cats. Methimazole does not inhibit the peripheral conversion of T4 to T3 and has no effect on thyroid hormones already formed or present in circulation.

Hyperthyroidism is a common endocrinopathy in older cats. Cats may present with clinical signs including polyphagia, weight loss, polyuria and polydypsia.
Methimazole is considered the antithyroid drug of choice for test treatment to normalize serum T4 concentration to assess the effect of resolving hyperthyroidism on renal function. If renal function improves or remains static, a more permanent therapeutic option, including surgical thyroidectomy or 131I, may be chosen. Methimazole should be used as a short-term therapeutic management of hyperthyroidism to prepare a cat for surgery or the prolonged hospitalization associated with 131I. Finally, methimazole can be used as long-term management of hyperthyroidism.