Is Thyroid Scanning Useful in Cats with Hyperthyroidism?

Thyroid scintigraphy (thyroid scanning by nuclear imaging) is a powerful imaging technique for visualizing thyroid tumors.  Only a few specialized facilities (such as the Animal Endocrine Clinic) have the equipment and special licensing required to perform scintigraphy.  In this week’s blog, I will be discussing how thyroid scintigraphy works, and its four main uses: diagnosing hyperthyroidism, locating impalpable tumors (tumors that cannot be felt on exam), diagnosing thyroid carcinomas, and calculating a radioiodine dose.

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.

Thyroid scintigraphy for diagnosing hyperthyroidism
Sometimes, even an endocrinologist will have trouble telling whether a cat is hyperthyroid.  Its T4 values may be on the borderline of abnormal, and the tumor may be difficult to palpate.  Thyroid scintigraphy is the “gold standard” for determining whether the cat is hyperthyroid.

In a normal cat, the thyroid gland will take up about as much of the tracer as the salivary glands do (see Figure 1).  On the scan, we expect the thyroid and salivary glands to be equally bright (a 1:1 brightness ratio).

Figure 1: Thyroid scan of a normal cat.  

Note that the salivary and thyroid glands are equally bright.

However, in a hyperthyroid cat, the adenoma cells take up far more of the tracer than the salivary glands do (see Figures 2 & 3).  Therefore, the adenomas show up far more brightly than the salivary glands.  This definitively diagnoses hyperthyroidism.  In order to diagnose hyperthyroidism in borderline cases, the specialist must use the software to carefully measure the densities of the thyroid and salivary glands.

Figure 2: Thyroid scan of a cat with unilateral right-sided thyroid adenoma.  

Note the brightness of the adenoma compared to the brightness of the salivary glands.

Figure 3: Thyroid scan of a cat with bilateral (two-sided) thyroid adenomas.  

Note the brightness of the adenomas compared to the brightness of the salivary glands.

Locating impalpable tumors
Impalpable tumors are fairly common in hyperthyroid cats; they affect about one-third of hyperthyroid cats.  These impalpable tumors come in two forms:  ectopic (not in the normal thyroid region) and large tumors that gravity pulls into the chest cavity.

Ectopic thyroid tissue is an embryological phenomenon that we see fairly often.  As the cells that develop into the thyroid gland traverse the embryo, they can leave traces of tissue anywhere from the base of the tongue to the base of the heart.   These traces can develop into functional thyroid tissue.  If these tissues develop a tumor, it will be impalpable, especially if the ectopic tumor is located on the tongue or in the chest cavity.  A thyroid scan can quickly and easily locate these tumors (see Figure 4).

Figure 4: Thyroid scan of a cat with an ectopic thyroid adenoma in the chest cavity. 

The veterinarian will not be able to palpate this tumor because of its location.

Like ectopic tumors, large tumors that descend into the chest cavity are impalpable.  We often see these tumors in cats that have been hyperthyroid for many months.  As the tumor grows progressively larger, gravity pulls progressively harder.  This causes the tumor to descend into the chest cavity.  Again, a thyroid scan swiftly detects these kinds of thyroid tumors (see Figure 5).

Figure 5: Thyroid scan of a cat with bilateral thyroid tumors. The left thyroid adenoma is small but should be palpable. The right thyroid tumor is very large and gravity has pulled it down into the thoracic cavity. The veterinarian would not be able to feel this tumor either.

Diagnosing thyroid carcinomas
As I discussed in a previous blog, some hyperthyroid cats have malignant thyroid carcinomas.  Neither palpation nor blood tests can determine whether a hyperthyroid cat has an adenoma or carcinoma.  Scintigraphy is extremely helpful in diagnosing invasive carcinomas.  On a scan, a carcinoma will appear as a large, irregularly shaped tumor extending beyond the limits of the normal thyroid region (see Figure 6).  We can also see whether the tumor has metastasized.

Figure 6: Thyroid scan of a cat with a thyroid carcinoma.  

Note that the tumor is irregularly shaped and is spreading into the chest cavity.

Diagnosing thyroid carcinomas is essential.  Without thyroid scintigraphy, a cat with a thyroid carcinoma would not receive the ultra-high dose of radioiodine necessary to destroy the cancer.

Calculating the radioiodine dose
In addition to diagnosing carcinomas (see above), thyroid scintigraphy helps us to make sure hyperthyroid cats receive the appropriate dose of radioiodine. Scintigraphy can precisely calculate the size of the tumor, even if it is impalpable.  With this information, we can calculate the dose of radioiodine for the cat, tailored specifically to that cat’s tumors.

Examples of recent thyroid scans
To see several examples of recent thyroid scans performed at our Bedford Hills facility, along with their accompanying interpretations, please visit our Nuclear Imaging Facebook page.