Associated Veterinary Specialists - Advanced Veterinary Care - St. Louis, MO
AVS Veterinary News

Hyperthyroidism in Cats

AVS has treated over 1000 cats with radioactive iodine that have been diagnosed with hyperthyroidism. Cats with "hyperthyroidism" actually have tumors of the thyroid gland that produce excessive amounts of normal thyroid hormone. They are mostly benign. They are rarely malignant but delay in definitive therapy (either radioactive iodine or surgery) may allow a benign tumor to become malignant. Radioactive iodine is the gold standard for therapy of these cats. When compared to other methods of treatment, radioactive iodine is by far the safest and most effective. Many of our clients have cost compared methimazole versus radioactive iodine and found that there is a break even point at about one and one-half years. Although radioactive iodine therapy has a high up front cost, methimazole has continuous, ongoing costs. The client incurs clinic visits, drug costs and costs of monitoring. Experts recommend that for the first two to three months of therapy with methimazole that a CBC, Platelet count, T4 and liver enzymes be monitored every two to three weeks and thereafter every three to six months. This plus frequent clinic visits adds up to very substantial cost. Also, methimazole has a high incidence of adverse effects, many of which preclude further treatment with the drug. An iodine deficient diet is manufactured by Hills, Y/D. This diet will reduce the output of thyroid hormone causing reduction of the hyperthyroid symptoms but does nothing to treat the thyroid tumor. The breakeven point for cost of Y/D compared to radioactive iodine is about 27 months. Either of these therapies frequently lose there efficacy after a period of time thus requiring the owner to seek radioactive iodine therapy after the cat has been treated for a long period with one of the other therapies. Any cost savings are then negated! The majority of our cats arrive with a diagnosis of hyperthyroidism. Therefore, we do a history and physical exam to confirm the diagnosis and help establish the radioactive iodine dose. Many of you are screening your feline patients during their annual exams and detecting hyperthyroidism almost as an incidental finding. This early detection makes the treatment quite easy since no secondary problems have had time to develop. Keep up the good work! When hyperthyroidism is suspected but not confirmed, we use several procedures to attempt to provide confirmation. A thyroid mass is palpable about 75 to 80% of the time which is additional evidence for the diagnosis. Repeat T4 tests may be helpful. If inconclusive consider a T3 suppression test or rarely thyroid scintigraphy. I personally believe that the incidence of cats developing renal failure after radioactive iodine therapy is exaggerated. When cats are screened with a biochemistry profile and urinalysis and found to be non-azotemic and have normally concentrated urine I have not had one develop renal failure that could be attributed to the iodine. Mild azotemia does not preclude them from treatment but provides a caution. When confronted with this situation, I recommend subcutaneous fluid therapy during the treatment and close monitoring of renal function. If the cat’s renal function is questionable, it is prudent to give a 2 week course of methimazole and follow up the renal function and T4 level to ensure that the cat can tolerate a reduction in T4. The published literature and our own experience shows that about 95% of cats treated with radioactive iodine will return to normal. A few percent will become hypothyroid and will require thyroid supplementation. Also, a few percent will remain hyperthyroid and will require a second dose of radioactive iodine. Our cats receive from 3.5 to 6.0 millicuries of radioactive iodine. They are given a filter-sterilized subcutaneous dose on Tuesday morning and are released on Thursday. Our staff monitors them every 4 hours day and night when they are hospitalized. We ask our clients to follow some specific guidelines about handling the cat for 2 weeks after therapy. The amount of exposure to radiation is very minimal but adherence to the guidelines will protect the client completely. We request that the client return to us with the cat in 2 weeks to check the radiation level with the Geiger counter. A month after treatment we request that the cat return to their referring veterinarian for follow up T4 and a copy be sent to us for our files. If you have any questions, please make and appointment with one of our internal medicine specialists at 314-739-1510 or 1-877-VET-SPEC.