Download Feline Hyperthyroidism: Updates on Diagnosis at MSU

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.