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FELINE HYPERTHYROIDISM: UPDATES ON DIAGNOSIS AT MSU Lukas Kawalilak, DVM, BSc Ag Diagnostic Imaging Resident Departments of Small and Large Clinical Sciences College of Veterinary Medicine Veterinary Medical Center A-174 Michigan State University East Lansing, MI 48824 Nuclear scintigraphy Thyroid scintigraphy is one of the most accurate ways to both quantitatively and qualitatively assess a suspect hyperthyroid cat.1 However, access to nuclear imaging equipment and the expertise needed to obtain a radioactive materials license typically limits the use of this diagnostic test to academic institutions. Scintigraphy uses a radioactive isotope injected into the blood stream that then emits gamma rays detected by a device called a gamma camera. These isotopes can be specifically targeted to a wide variety of the body’s MSU’s gamma camera has a wide range of tissues, including the thyroid. When evaluating feline motion allowing for scanning of sedated patients with suspect hyperthyroid disease, scintigraphy small and large animals allows us to determine unilateral vs. bilateral disease, check for metastatic disease and ectopic tissue, as well as evaluate efficacy of radioiodine therapy.2 Which isotope to use? When imaging the thyroid gland using scintigraphy, there are two radioisotopes that can be used.3 Iodine-123 is an available gamma emitter that mimics the uptake of non-radioactive iodine in the body, but its long half-life, higher gamma energy emission, and increased cost severely limits its use in veterinary medicine. The most common thyroid radiopharmaceutical is pertechnetate (TcO4-) which is similar in size and charge to iodine, as well as readily available and inexpensive. This isotope is unique in that it is rapidly taken up by the follicular cells of the thyroid gland, causing it to concentrate in this specific organ, but not subsequently organified into thyroid gland products T3 and T4. This means that a pertechnetate thyroid scan can be performed as soon as 20 minutes after injection of the radiopharmaceutical, much quicker than if I-123 was used. Unlike pertechnetate, I-123 is organified by the thyroid, making it the isotope of choice when determining if a dose of I-131 will remain in the thyroid long enough for effective destruction of thyroid cells. Factors affecting thyroid scintigraphy Increases Thyroid Uptake Decreases Thyroid Before we perform the scan there are Uptake a variety of factors that have the Methimazole Foods with high iodine potential to either increase or content (seaweed, fish, offal) decrease the uptake of pertechnetate TSH supplementation Iodinated contrast by the thyroid. Taking a thorough media history is key in identifying these T3 / T4 supplementation factors so the results of the scintigraphic scan are not misinterpreted. The most common confounding factor we encounter at MSU is the use of methimazole. This drug blocks the production of T3 and T4, causing increased levels of thyroid stimulation hormone and subsequent increased uptake of both iodine and pertechnetate.4 Clinically, this means that cats must be off of methimazole for at least 10 days prior to scintigraphy as the increased uptake would result in an inaccurate scan.5 Performing the scan The cat is first injected with a dose of 1-3 mCi (37148MBq) of NaTcO4. After 20 minutes, the cat is then heavily sedated and placed on the surface of the gamma camera. VD, left, and right lateral projections of the head and neck, as well as the thorax are obtained. Each projection takes approximately 1 minute to obtain. As the camera is ‘counting’ the number of gamma rays being emitted from the patient, heavy sedation is key to this study; any patient motion will result in a blurry, inaccurate image. Common scintigraphic findings Normal: The appearance of the normal feline thyroid scan is characterized by uniform distribution of radioactivity throughout both thyroid lobes (arrows). These should appear as elongated ovals, symmetrical in size and position in the central cervical region. The thyroid lobe margins should be smooth and regular with no ectopic tissue present. The zygomatic salivary gland is another organ that takes up and secretes pertechnetate in cats, and should also have uptake (arrowheads). There is lesser uptake in the parotid and mandibular glands. The ratio between either lobe of the thyroid and zygomatic salivary gland should be 1:1.6,7 Abnormal: 4 patterns of abnormal uptake have been identified in cats.8 All are indicative of the presence of feline hyperthyroidism. A – Unilateral increased uptake, with a suppressed contralateral lobe (normal tissue). This is the 2nd most common result of scintigraphy, seen in 32% of cats. B – Bilateral, asymmetric uptake, indicating abnormal tissue in both thyroid lobes. This is seen in 52% of cats. C – Bilateral, symmetric uptake. This is seen in 12% of cats. D – Multifocal disease, indicating metastases, ectopic tissue, or both. This is the rarest result of scintigraphy, seen in 4% of cases. Image from Veterinary Radiology and Ultrasound 2015 56 (1) 84-95 Thyroid scintigraphy in 2096 cats with hyperthyroidism 98.5% of cats with a thyroid to salivary gland ratio of greater than 1.5 were confirmed to have hyperthyroidism on bloodwork, making this an ideal test for identifying cats with this disease. References/additional resources 1. Daniel GB, Neelis DA. Thyroid scintigraphy in veterinary medicine. Semin Nucl Med 2014;44:24–34. 2. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clin Tech Small Anim Pract. 2006;21:10–16. 3. Feeney DA, Anderson KL. Nuclear Imaging and Radiation Therapy in Canine and Feline Thyroid Disease. Vet Clin North Am - Small Anim Pract. 2007;37:799–821. 4. Fischetti AJ, DiBartola SP, Chew DJ, Schenck PA, Meadows C. Effects of methimazole on thyroid gland uptake of 99MTC- pertechnetate in 19 hyperthyroid cats. Vet Radiol Ultrasound. 2005;46:267–272. 5. Peterson ME, Guterl JN, Rishniw M, Broome MR. Evaluation of quantitative thyroid scintigraphy for diagnosis and staging of disease severity in cats with hyperthyroidism: Comparison of the percent thyroidal uptake of pertechnetate to thyroid-to-salivary ratio and thyroid-to-background ratios. Vet Radiol Ultrasound. 2016;57:427–440. 6. Bettencourt A, Daniel GB, Panciera D, Larson M, Werre SR. Evaluation of thyroid to background ratios and comparison of various scintigraphic measurements and their correlation to serum t4 in hyperthyroid cats. Vet Radiol Ultrasound. 2016;57:290–298. 7. Wallack S, Metcalf M, Skidmore A, Lamb CR. Calculation and Usage of the Thyroid To Background Ratio on the Pertechnetate Thyroid Scan. Vet Radiol Ultrasound. 2010 Sep 10;51:554–560. 8. Peterson ME, Broome MR. Thyroid scintigraphy findings in 2096 cats with hyperthyroidism. Vet Radiol Ultrasound. 2015;56:84–95.