How We Use Thyroid Scintigraphy as a Diagnostic Tool

Thyroid scintigraphy (thyroid scan) as a diagnostic tool:

Thyroid scintigraphy is a very sensitive and safe method to evaluate the anatomy and function of the thyroid gland of hyperthyroid cats. It is a quick scan (about 3 minutes), relatively stress-free procedure for the patients, and does not require sedation in most cases. Sodium pertechnetate 99mTcO4- is administered by an injection. About 20-60 minutes after injection, thyroid images are obtained with the cat lying on its belly and then its side. Scintigraphy delivers useful and detailed information about the thyroid gland in cats confirmed with or suspected to have hyperthyroidism, such as:

1. Confirm hyperthyroidism. Thyroid scintigraphy is the gold standard diagnostic test for cats with mild hyperthyroidism. Scintigraphy allows us to “see” and measure the activity level of the thyroid, even mildly hyperthyroid cats will have increased activity within their thyroid but normal cats will not.
2. Rule out hyperthyroidism in cats with high thyroid hormone levels due to other illness or false-positive test results. In some cats with mild hyperthyroidism, thyroid hormone levels may be inconsistently elevated leading to uncertainty about the diagnosis. Also other illnesses may cause high thyroid hormone levels leading to an inaccurate diagnosis of hyperthyroidism. Since thyroid hormone levels can be unreliable in some cases (i.e. not every cat with high T4 and/or high free T4 level has hyperthyroidism), thyroid scan is useful to determine if treatment is truly indicated.
3. The presence of bilateral or unilateral disease. One study has shown that hyperthyroid cats with bilateral disease are at a higher risk of iatrogenic hypothyroidism after I-131. Knowing this information prior to I-131 therapy can significantly affect the dose of I-131 given to that patient to minimize this risk.
4. The presence of ectopic tissue. Palpation may underestimate the size of the thyroid nodule if ectopic tissue is present and therefore also underestimate the dose of I-131 required to resolve the disease. A recent study evaluating thyroid scintigraphy in almost 1000 hyperthyroid cats revealed some cats with multiple areas of hyperactive thyroid tissue. Ectopic tissue can occur anywhere from the base of the tongue to the heart.
5. The size of the thyroid nodule. Thyroid volume is considered an important factor affecting the outcome after I-131 in humans and cats. Thyroid volume can be assessed via scintigraphy to positively influence the outcome of I-131.
6. If thyroid cancer is suspected or if the nodule is due to benign disease.
7. If a cervical mass is due to abnormal thyroid tissue or non-thyroid tissue. Cervical masses in cats may also occur because of a parathyroid mass or cyst, abscess, or other kinds of tumors.

While thyroid scintigraphy is not mandatory before I-131 therapy, it is now considered a “best practice” strategy for the management of hyperthyroid cats. Thyroid scintigraphic findings result in I-131 dose adjustments specific for each patient, confirm the diagnosis of hyperthyroidism in equivocal cases, and offers prognostic data in cases of carcinoma. Thyroid scintigraphy provides vital information that is not readily accessible through other forms of clinical testing.

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