In response to the recent misinformation being distributed about the value of thyroid scintigraphy when treating hyperthyroid cats with radioiodine, we felt it was important to remind local practitioners of the following information. Thyroid scintigraphy is a nuclear medicine procedure that produces a visual display of functional thyroid tissue based on the selective uptake of various radionuclides by thyroid tissue. Thyroid scintigraphy provides valuable information regarding both thyroid anatomy and physiology and can play an integral role in the diagnosis and management of cats with hyperthyroidism. The procedure is extremely safe, does not require the use of anesthesia and is cost effective, especially when considering the costs of an incorrect diagnosis or inappropriate treatment. Scintigraphy has been an invaluable tool in studies evaluating the pathogenesis of hyperthyroidism and continues to be helpful as the incidence of the disease continues to increase and the factors responsible for the disorder continue to be elucidated. The imaging and activity data provided by thyroid scintigraphy complements the hormonal evaluation of our patients. By allowing the direct visualization of the adenomatous thyroid tissue responsible for the development of hyperthyroidism, thyroid scintigraphy allows the diagnosis of hyperthyroidism before laboratory tests are consistently abnormal. Perhaps even more importantly, thyroid scintigraphy can also exclude a diagnosis of hyperthyroidism in cats with thyroid hormone elevations of unknown clinical significance. We know from studies involving large number of cats that up to 6-7 % of the senior cat population will have TT4 levels in the upper 50% of the reference range and elevated fT4ED concentrations and are found not to be hyperthyroid based on scintigraphic findings. Therefore, not every cat with a total T4 and/or fT4ED above the reference range is hyperthyroid. Facilities that perform radioiodine therapy without confirming the diagnosis via thyroid scintigraphy will ultimately administer radioiodine to cats that are not hyperthyroid, potentially rendering them permanently hypothyroid. Thyroid scintigraphy also provides an additional method for determining the relative severity of thyroid disease that is less affected by the presence of concurrent nonthyroidal illness than laboratory evaluations. Recent research confirms that iatrogenic hypothyroidism in cats following radioiodine therapy contributes to the development of azotemia and shortened survival times. To minimize the incidence of iatrogenic hypothyroidism post radioiodine therapy the lowest effective dose should be administered. In an effort to administer the lowest radioiodine dose possible, the volume of adenomatous thyroid tissue present in the individual hyperthyroid cat should be considered. Thyroid scintigraphy provides an excellent method for evaluating the size of the hyperfunctional thyroid tissue and is capable of determining the presence of ectopic or intrathoracic thyroid tissue. A recent study found that up to 20% of hyperthyroid cats had functional, ectopic, extra-thyroidal tissue. This is a large increase in the percentage of cats found to have extra-thyroidal uptake compared to earlier studies and may represent a fundamental change in the pathogenesis of the disease. These results highlight the importance of scintigraphy as the results may impact treatment decisions such as the role of surgery and determining the dose of radioiodine that will be necessary to successfully treat the disease. Thyroid scintigraphy also provides valuable information in the diagnosis and evaluation of hyperthyroid cats with thyroid carcinoma. At this years NAVC meeting, data was presented showing that cats in older age groups, especially those that had received long term anti-thyroid medications, had an increase in thyroid scans consistent with carcinoma. The diagnosis of thyroid carcinoma can be challenging (even with histopathology) but without pre-treatment scanning these cases would go undetected. Despite the value of the information obtained by performing thyroid scintigraphy, it is not a mandatory requirement prior to radioiodine therapy. Just like thoracic radiographs are not required prior to amputating a limb in a patient with a primary bone tumor or preanesthetic laboratory screening is not required prior to dental prophylaxis or other elective procedures requiring anesthesia, thyroid scintigraphy is not required prior to radioiodine therapy. Never-the-less the utilization of scintigraphy is considered good medicine as the findings can and do result in modification of the therapeutic plan, aid in the diagnosis of hyperthyroidism and provide prognostic information. Indeed, in light of the numerous reasons to perform thyroid scintigraphy before radioiodine therapy, the real question of why not to perform thyroid scintigraphy prior to radioiodine therapy needs to be asked. We believe that good medicine should dictate the management of our feline hyperthyroid patients rather than the business model of the facility performing the radioiodine therapy. Doing what is best for the patient, the client and our referring veterinarians is our goal and scintigraphy helps us meet that goal. If you have any questions or comments please do not hesitate to contact us at anytime. Sincerely, Michael R. Broome, DVM, MS, DABVP Advanced Veterinary Medical Imaging David Bruyette, DVM, DACVIM Medical Director VCA West Los Angeles Animal Hospital Associate Clinical Professor Department of Radiation Oncology Ronald Reagan School of Medicine University of California - Los Angeles --------------------
  1. Peterson M.E., Melian C. and Nichols R., Measurement of Serum Concentrations of Free Thyroxine, Total Thyroxine, and Total Triiodothyronine in Cats with Hyperthyroidism and Cats with Nonthyroidal Disease. J Am Vet Med Assoc, 2001. 218(4): p. 529-36.
  2. Harvey A.M., Hibbert A., Barrett E.L., et al., Scintigraphic Findings in 120 Hyperthyroid Cats. Journal of Feline Medicine & Surgery, 2009. 11(2): p. 96-106.
  3. Williams T.L., Elliott J. and Syme H.M., Association of Iatrogenic Hypothyroidism with Azotemia and Reduced Survival Time in Cats Treated for Hyperthyroidism. J Vet Intern Med, 2010. 24: p. 1086-1092.