|Dr. Mark Peterson performing a thyroid scan on one of his hyperthyroid cat patients|
Thyroid scintigraphy is considered the “gold standard” for diagnosing mild hyperthyroidism in cats (1-3, 6,7). It is considered to be the most sensitive diagnostic test available — yes, even better than determination of serum T4, free T4, or TSH for diagnosis of hyperthyroidism.
In addition, thyroid scanning is an invaluable tool for evaluating the stage and extent of thyroid tumors (adenomas and carcinomas) in hyperthyroid cats (1,4,5,8).
How Thyroid Scintigraphy Works
To perform thyroid scintigraphy, the specialist injects the cat with a small dose of a radioactive tracer subcutaneously. Over the next hour, the cat’s salivary glands and thyroid glands take up the tracer. The radioactive tracer, now in the salivary and thyroid glands, then emits gamma rays (a high energy electromagnetic wave, a bit stronger than an X-ray), which are detected by a gamma camera to form an image.
To perform thyroid imaging, the cats sits normally on the camera (ventral view) or lies on his or her side (lateral view) while the gamma camera acquires the thyroid image (Figure1, above). The scanning process itself generally takes less than a minute and does not require sedation.
To watch video of Dr. Peterson performing a thyroid scan on a cat with hyperthyroidism, click on this link.
Use of Thyroid Scintigraphy as a Diagnostic Test
In normal cats, the thyroid gland appears on thyroid scans as two well-defined, focal (ovoid) areas of radionuclide accumulation in the cranial to middle cervical region. The two thyroid lobes are symmetrical in size and shape and are located side by side (Figure 2).
|Figure 2: Thyroid scan of a normal cat. |
Notice the similar uptake of the radionuclide in the thyroid lobes and the salivary glands.
|Figure 3: Thyroid scan of a hyperthyroid cat with a single thyroid adenoma.|
Notice the increased uptake of the radionuclide in the thyroid tumor compared with the salivary glands.
|Figure 4: Thyroid scan of a hyperthyroid cat with bilateral thyroid adenomas.|
Notice the increased uptake of the radionuclide in both thyroid tumors compared with the salivary glands.
More Uses for Thyroid Scintigraphy
Thyroid scintigraphy has four more uses in hyperthyroid cats, other than as a diagnostic test. First, it is an excellent method for evaluating the size of ectopic thyroid tissue, which can be located anywhere from base of the tongue to the heart (Figure 5). Second, it can locate large tumors that gravity has pulled into the thoracic cavity. Finally, thyroid scintigraphy also provides valuable information for diagnosing and evaluating cats with thyroid carcinomas (Figure 6).
|Figure 5: Thyroid scan of a hyperthyroid cat with ectopic thyroid adenoma located with the chest cavity. Because of its location, this tumor could not be palpated on physical examination.|
|Figure 6: Thyroid scan of a hyperthyroid cat with a thyroid carcinoma (cancer). Note the extension of tumor beyond the limits of the normal thyroid capsule. This represents regional metastasis characteristic of carcinoma.|
Why Isn't Thyroid Scintigraphy Used More Often?
Apart from expense and the licensing needed to handle and administer radioisotopes, few veterinarians have access to the nuclear medicine equipment needed to obtain thyroid images or perform thyroid uptake determinations. Only a few large referral hospitals in the country offer nuclear scintigraphy to do thyroid scanning.
At the Animal Endocrine Clinic, we routinely perform thyroid scintigraphy on all of our cats in which the diagnosis is not completely clear cut (go to my website for more information). We believe that thyroid scintigraphy plays an essential role in the diagnosis and management of cats with hyperthyroidism.
- Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:10-16.
- Daniel GB, Sharp DS, Nieckarz JA, et al. Quantitative thyroid scintigraphy as a predictor of serum thyroxin concentration in normal and hyperthyroid cats. Veterinary Radiology & Ultrasound 2002;43:374-382.
- Forrest LJ, Baty CJ, Metcalf M.R, et al, Feline hyperthyroidism: Efficacy of treatment using volumetric analysis for radioiodine dose calculation. Veterinary Radiology & Ultrasound, 1996;37:141-145.
- Harvey AM, Hibbert A, Barrett EL, et al. Scintigraphic findings in 120 hyperthyroid cats. Journal of Feline Medicine and Surgery 2009;11:96-106.
- Kintzer PP, Peterson ME. Nuclear medicine of the thyroid gland. Scintigraphy and radioiodine therapy. Veterinary Clinics of North America: Small Animal Practice 1994;24:587-605.
- Mooney CT, Thoday KL, Nicoll JJ, et al. Qualitative and quantitative thyroid imaging in feline hyperthyroidism using technetium-99m as pertechnetate. Veterinary Radiology & Ultrasound 1992;33,313-320.
- Nap AM, Pollak YW, van den Brom WE, et al. Quantitative aspects of thyroid scintigraphy with pertechnetate (99m TcO4) in cats. Journal of Veterinary Internal Medicine 1994;8:302-303.
- Peterson ME, Becker DV. Radionuclide thyroid imaging in 135 cats with hyperthyroidism. Veterinary Radiology 1984;25:23-27.