Thyroid Imaging for Preoperative Staging of Hyperthyroid Cats

Thyroid scintigraphy (thyroid scanning or thyroid imaging) provides valuable information regarding both thyroid anatomy and physiology and can play an integral role in the staging and management of thyroid disease in cats with suspected hyperthyroidism.

Ideally, thyroid scintigraphy would be performed in all hyperthyroid cats prior to surgical thyroidectomy to identify the extent and location of the cat's hyperfunctioning thyroid tumor tissue (1-7).  In hyperthyroid cats, thyroid scintigraphy directly visualizes all functional thyroid tissue and will easily reveal whether the hyperthyroid cat has a thyroid tumor affecting only one lobe (unilateral disease) or both lobes (bilateral disease). See the scans in Figure 1 below.

Fig. 1: Thyroid scintigraphy in 3 hyperthyroid cats.
The cat on the left has a single thyroid tumor (Unilateral), whereas the cat on the right has 2 thyroid tumors that involve both thyroid lobes equally (Bilateral symmetric). The cat in the middle panel also has bilateral thyroid disease, but it is possible that the surgeon could mistake the smaller thyroid lobe as normal since the thyroid tumor enlargement is so asymmetric.

Some hyperthyroid cats develop very large thyroid tumors that descend into the chest cavity. making them difficult to palpate or identify at surgery. We often see these tumors in cats that have been hyperthyroid for many months to years (8,9). As the tumor grows progressively larger, gravity pulls progressively harder. This causes the tumor to descend into the chest cavity. Again, thyroid scintigraphy will detect these intrathoracic thyroid tumors that could easily be missed at surgery (Figure 2).

Fig. 2: Thyroid scintigraphy in a hyperthyroid cat with bilateral thyroid tumors.
In this cat, the larger thyroid tumor has descended into the chest cavity because of the effects of gravity. Without thyroid scintigraphy, this tumor could easily be missed at surgery, resulting in persistent hyperthyroidism.

Ectopic thyroid tissue is an embryological phenomenon that we see in about 3-5% of all hyperthyroid cats (8,9).  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 thyroid remnants can develop into functional thyroid tissue.  If such ectopic thyroid tissue develops a tumor, it will not be palpable or easily found at surgery, especially if the ectopic tumor is located on the tongue or in the chest cavity.  Thyroid scintigraphy can quickly and easily locate these tumors (see Figure 3).

Fig. 3: Thyroid scintigraphy in a hyperthyroid cat with a single ectopic thyroid nodule.
In this cat, no thyroid tumors could be palpated in the neck area where the thyroid gland is normally located. The large thyroid tumor has arisen from ectopic thyroid tissue that is located within the chest cavity. Without thyroid scintigraphy, this tumor could easily be missed at surgery, resulting in persistent hyperthyroidism.

Finally, thyroid scintigraphy also provides valuable information for diagnosing and evaluating cats with thyroid cancer (carcinomas) and metastasis.  With scintigraphy, a carcinoma will appear as a large, irregularly shaped tumor extending beyond the limits of the normal thyroid region (see Figure 4).  We can also see whether the tumor has metastasized or shows local invasion of the surrounding tissues. Thyroid carcinomas account for 2-3% of all thyroid tumors in cats (8,9).

Fig. 4: Thyroid scintigraphy in 6 hyperthyroid cats with a thyroid carcinoma (thyroid cancer).
In these cats with thyroid carcinoma, notice that the thyroid tumors are very large, multinodular, with extension and metastasis into the chest cavity. Surgery is unlikely to be successful in any of these cats, since these cancers are highly vascular and invasive, and it would be difficult to find all malignant thyroid tissue at surgery.

If thyroid scintigraphy is not available, the surgeon will have to make a decision upon what thyroid tissue to remove based on the visual appearance of the glands during surgery. In some cases, the surgeon may be fooled into leaving abnormal functioning thyroid tissue behind, especially if located in the chest cavity. Failure to identify and remove all hyperfunctional tissue will result in failure to cure the cat's hyperthyroidism.

Why Isn't Thyroid Scintigraphy Always Done Prior to Thyroidectomy?

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. However, if available, I believe that thyroid scintigraphy can play an essential role in the preoperative staging and surgical management of cats with hyperthyroidism.

References:

1. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; 199-203.
2. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;571-592.
3. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
4. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
5. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
6. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
7. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21,10-16. 
8. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012;26:754.
9. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. Congress Proceedings, 22^nd ECVIM-CA Congress (The European College of Veterinary Internal Medicine – Companion Animals) 2012; 224