Do Hyperthyroid Cats Always Need a Thyroid Scan? Your questions answered.
Q1: What about a thyroid scan? Is this needed in my cat? Do you do this procedure at Hypurrcat?
Thyroid scintigraphy provides valuable information regarding both thyroid anatomy and physiology and can play an integral role in the diagnosis, staging, and management of hyperthyroidism in cats.
At Hypurrcat, we perform thyroid imaging on all cats in order to take a picture of their thyroid tumor(s) as part of their diagnostic workup. This is an essential diagnostic test, and we believe that no cat should ever be treated with I-131 without first having such a scan.
Q2: How does a thyroid scan work in a hyperthyroid cat to locate the thyroid tumor(s)?
After administering a short-acting radioactive isotope (radionuclide) that concentrates in thyroid tissue, thyroid imaging directly visualizes the normal thyroid gland, as well as the small tumor(s) responsible for hyperthyroidism in cats. Because this procedure utilizes the physiology of the thyroid gland to create an image, thyroid imaging is so sensitive that it can actually demonstrate the presence of overactive thyroid tumors even before the serum T4 and T3 concentrations become high. Unlike other imaging techniques, thyroid imaging assesses thyroid function as well as just structure and anatomy.
To perform a thyroid scan, a small radionuclide dose is administered intravenously. One hour later, the cats are laid on their abdomen (ventral view) or side (lateral view) while the gamma camera acquires the thyroid image. The scanning process itself only takes about 3 minutes and generally is done without use of any sedation.
Preparing a Feline Patient for a Thyroid Scan by Injecting Radioactive Dye
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Performing a Thyroid Scan on a Hyperthyroid Cat
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Thyroid Tumors in Cats: Benign vs Malignant
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Learn more about thyroid scans
Click here to read Dr. Peterson’s blog posts about thyroid scans.
Q3: Why does my cat need a thyroid scan? We already know that he/she is hyperthyroid!
There are many reasons to do a thyroid scan to help evaluate a hyperthyroid cat before radioiodine treatment.
1. First of all, thyroid scintigraphy (thyroid imaging or thyroid scanning) is the gold standard test to confirm the diagnosis. As opposed to standard serum thyroid function tests (T4, T3, and free T4), this is a much more accurate test, as documented in Dr. Peterson’s research paper published in the journal Veterinary Radiology & Ultrasound.
2. Thyroid scans will identify your cat’s thyroid tumor or tumors, no matter where the tumor tissue is located. The scan will detect tumor tissue that has invaded into the chest or metastasized to another part of the body, so we can easily diagnose and rule out thyroid cancer. Although extremely rare in recently diagnosed cats, our studies find that the prevalence of thyroid cancer may increase to over 25% in cats treated medically for over 4 years.
3. Another important reason to do scintigraphy is to allow Dr. Peterson to accurately determine your cat’s thyroid tumor volume. Obviously, cats with larger amounts of tumor tissue will need higher doses of radioiodine (see figure below), but the severity of hyperthyroid disease frequently cannot be adequately determined with thyroid blood tests alone. Thyroid scanning is the only way to determine the volume of thyroid tumor tissue that your cat has that needs to be destroyed.
Most recently diagnosed hyperthyroid cats that we treat will require very low doses of radioiodine (<2 mCi). This is an ultra-low dose, compared to the higher doses of radioiodine of 3.5-5 mCi generally administered to all cats by treatment facilities that do not do thyroid scans to measure the tumor volume.
Why is this important? Lowering the 131-I dose lowers your cat’s whole body radiation exposure; although radioiodine is good in that is destroys the thyroid tissue. Why give larger amounts of radiation if not needed to accomplish this goal?
Equally important, low-dose radioiodine will lower the prevalence of post-treatment hypothyroidism (underactive thyroid), which is common when we give an overdose of radioiodine to cats. Unlike other treatment facilities, we continue to closely monitor your cat’s thyroid function after treatment, both with a complete panel of thyroid tests and a repeat thyroid scan, when needed. See our research page that details our studies of use of all complete thyroid panel for monitoring of cats with hyperthyroidism, all of which are done at no charge to you if you decide to enroll.
4. Thyroid scintigraphy should always be done in hyperthyroid cats when a thyroid tumor is not palpable, especially in cats with severe or long-standing hyperthyroidism. In many of these cats the thyroid tumor has fallen into the chest. The only way to determine the proper dose of radioiodine in these cats is to calculate the tumor volume by thyroid scanning.
Learn more about thyroid scans
Click here to read Dr. Peterson’s blog posts about thyroid scans.
Q4: Is anesthesia or sedation required for the thyroid scan?
Your veterinarian or someone at another treatment facility might have told you that they do not recommend thyroid scanning because of the anesthesia that is required to do this scan. They are correct is worrying about the danger of anesthetizing a cat with hyperthyroidism, but this is not a reason to avoid use of thyroid scintigraphy.
We never anesthetize our hyperthyroid cats and only very rarely use even mild sedation to complete a thyroid scan. Over 99% of the cats we scan can be easily done with no sedation at all.
See this video of Dr. Peterson doing a thyroid scan:
Performing a Thyroid Scan on a Hyperthyroid Cat
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