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What should you expect from the facility
providing radioiodine therapy for your hyperthyroid cat?

In a word, confidence....

Confidence in the diagnosis:
Hyperthyroidism is caused by one or more small and usually benign tumor(s) in the thyroid gland1,2. These tumors function autonomously to produce excessive thyroid hormone levels. The diagnosis of hyperthyroidism is usually straightforward and is typically made with the combination of historical symptoms, physical findings and laboratory evaluation3. Generally, a significant elevation in the T4 (thyroxine) level, in conjunction with appropriate historical and physical findings is sufficient to confidently allow a diagnosis of hyperthyroidism.
Unfortunately, not all cats with hyperthyroidism have an elevated T4. Sometimes cats with early or mild hyperthyroidism have normal T4 levels4. As a result other tests have been described that allow the diagnosis of hyperthyroidism in cats with early or mild disease5,6. Currently the non-protein bound or “free” T4 as measured by a methodology called equilibrium dialysis (fT4ed ) is considered the most sensitive laboratory test for the diagnosis of hyperthyroidism7. Unfortunately, this test is NOT perfect either. Between 6% and 12 % of the cats with elevated fT4ed values are not hyperthyroid.
A procedure called thyroid scintigraphy (a diagnostic imaging procedure) has long been considered the “gold standard” in the diagnosis of feline hyperthyroidism8. This procedure allows the direct visualization of the thyroid gland and the ability to see the small tumor(s) responsible for hyperthyroidism in cats. This procedure utilizes the physiology of the thyroid gland to create an image and is so sensitive that it can actually demonstrate the presence of these tumors long before they become clinically significant or result in laboratory value abnormalities.
At Advanced Veterinary Medical Imaging, thyroid scintigraphy is performed before every radioiodine therapy to ensure the accuracy of the diagnosis.

Confidence in the radioiodine dosing methodology:
Numerous methods for determining the radioiodine dose for cats with hyperthyroidism have been described in the veterinary literature9,10,11,12,13,14. These methods are quite variable but include 2 basic categories, namely 1.) individual or “patient specific” dose determination and 2.) fixed or “one size fits all” dosing.
The patient specific dosing methodologies attempt to maximize the success of the therapy by taking into consideration a number of patient specific variables. These variables include the size of the thyroid tumor, the degree of functional autonomy demonstrated by the tumor and several specific physiologic parameters including biologic half-life and iodine uptake15,16,17,18.
The most accurate way to determine the extent of the thyroid tumor is with thyroid scintigraphy. Some methods of radioiodine dose determination rely on the physical palpation of the thyroid gland alone to estimate tumor size. Unfortunately, a significant percentage of thyroid adenomas become intrathoracic secondary to the chronic effects of gravity on the progressively enlarging mass. Another group of cats develop hyperthyroidism secondary to adenomas of ectopic, often intrathoracic, thyroid tissue (see figure 3). Both of these situations render these adenomas beyond the reach of palpation. Furthermore, a small percentage of cats develop hyperthyroidism secondary to functional thyroid carcinomas. The full extent of these tumors is often grossly underestimated by physical palpation alone.
The fixed dose approach assumes that most patients can be successfully treated by administering the same “fixed” dose to each patient. This method disregards the importance of the individual parameters described above. To accomplish a reasonable success rate, this method utilizes an above average dose that is administered to every cat. As a result, a large number of cats treated using this method receive excessive amounts of radioiodine exposing both the patient and veterinary personnel to unnecessary levels of radiation.
At Advanced Veterinary Medical Imaging, thyroid scintigraphy is performed before every radioiodine therapy to aid in determining the appropriate dose of radioiodine for each individual cat.

Confidence in the ongoing care provided for your cat by the facility and staff during the mandatory hospitalization associated with the procedure:
The care needed by cats undergoing radioiodine therapy is usually minimal. The radioiodine treatment itself is not associated with any reported side effects. Never the less the average age of the cat’s with this disease is 13 years. The effects of the chronic thyroid hormone excess on other organ systems in the body, especially the cardiovascular system, can be significant19,20,21. Major concurrent conditions necessitating ongoing therapy are common in these patients. Common concurrent conditions include but are not limited to diabetes mellitus, congestive heart failure, renal insufficiency, and inflammatory bowel disease.
Some facilities that perform radioiodine therapy rely on patient care and monitoring provided by veterinary technicians who report to off site veterinarians regarding the condition of the hospitalized patients. Furthermore, some radioiodine therapy facilities utilize the services of out of state consultants to perform the actual radioiodine therapy. These out of state consultants fly in for the few hours necessary to administer the radioiodine treatment and then leave the patients in the care of individuals unlicensed to administer radioactive materials. Needless to say, this approach is a cause for great concern among many licensed radioactive materials users.
At Advanced Veterinary Medical Imaging all patients receiving radioiodine therapy are evaluated, treated and then supervised for the duration of their hospital stay by Dr. Michael Broome. Dr. Broome is both a pioneer and recognized expert in the use of radioactive iodine for the treatment of feline hyperthyroidism. Following the completion of a Master’s project in comparative thyroid physiology at U.C. Davis, Dr. Broome became the first veterinarian in a private practice in the United States to receive a license for the use of radioactive iodine in the treatment of feline hyperthyroidism. Dr. Broome has authored numerous articles on the topic and personally cared for over 10,000 cats with hyperthyroidism using radioiodine. Dr. Broome is regularly consulted by the California Department of Health regarding issues of radiation safety relating to the use of radioiodine in animals.

Thyroid Scintigraphy

To see more examples of thyroid scintigraphy with additional description see the thyroid scintigraphy page.



Figure 1. Thyroid scintigraphy of a cat with normal thyroid function. Note the symmetric appearance of the thyroid lobes.

Figure 2. Thyroid scintigraphy of a cat with early hyperthyroidism secondary to a small left sided thyroid adenoma.

Figure 3. Thyroid scintigraphy of a cat with hyperthyroidism secondary to an intrathoracic ectopic thyroid adenoma. This mass was NOT palpable on physical examination.

Figure 4. Thyroid scintigraphy in a cat with hyperthyroidism secondary to bilateral asymmetric adenomatous thyroid hyperplasia.

Figure 5. Thyroid scintigraphy in a cat with hyperthyroidism secondary to bilateral symmetric adenomatous thyroid hyperplasia.

Figure 6. Thyroid scintigraphy in a cat with marked hyperthyroidism secondary to thyroid carcinoma. Notice the extension of the disease beyond the normal extession of the thyroid capsule. This represents regional metastasis.


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