How to Best Manage Non-functional Thyroid Tumors in Cats

I have a 3½ year old castrated male DLH that was found to have a 1-cm mass in the region of the left thyroid gland on a wellness exam 10 months ago. The owner declined cytology at that time, and on followup last week the mass has now grown to ~2-cm (22mm) in size. This is an incidental finding on physical exam, and the cat is maintaining his weight and otherwise seems normal at home. The body condition score, muscle condition score, and heart rate are all normal in this cat.

Routine lab work, including a CBC, serum chemistry profile, and urinalysis were all normal. Results of a serum concentrations of total T4 (2.5 µg/dl; normal, 0.8-4.0 µg/dl) and free T4 (26 pmol/L; normal, 10-50 pmol/L) were also well within reference range limits. The results of my aspiration cytology of the mass is as follows:

• The slides are of moderate to high cellularity containing scattered clusters of epithelial cells in a background of peripheral blood and scattered bare round nuclei. The epithelial cells display round to oval nuclei with a moderate amount of encircling, indistinct, pale basophilic cytoplasm. These cells display mild anisocytosis and anisokaryosis. No infectious organisms are observed. 
• This is an epithelial cell tumor and cytologically it appears to be benign. This is likely a thyroid gland tumor; if so, the majority of thyroid tumors in cats are benign adenomas. Rarely, thyroid carcinomas occur in cats and are very difficult to distinguish from adenomas cytologically. Histopathology is required to specifically identify the origin of these epithelial cells. It should be emphasized that other epithelial cell tumors other than thyroid tumors in this area have similar cytologic appearance.

I spoke with a consultant at the lab, who recommended just monitoring the mass rather than surgical removal, as it appears to be benign. The owner doesn't want surgery unless the mass is likely to cause the cat problems in the future. 

Is it reasonable to just monitor the cat rather than treat? I've heard about enormous thyroid masses that can become cystic and develop into thyroid carcinoma so I don't want that to happen. While we can certainly monitor for growth and deal with the mass if it gets larger, I'd like to remove it now if it's pretty much guaranteed to continue to grow and has potential to cause issues in the future.

Thanks for your advice!

My Response:

First of all, as the pathologist indicates, we can not make a definitive diagnosis of thyroid adenoma based on cytology alone. If this is thyroid adenoma, the tumor will likely continue to grow with time (1,2), as you have already seen in this case. It's also possible for an adenoma to become cystic (3), which can make treatment much more complicated.

Finally, in addition to tumor growth, it is also possible to see transformation from thyroid adenoma into carcinoma with time (2,4,5). Based on my studies, this would most likely occur in cats with long-standing hyperthyroidism treated with methimazole (longer than 3-4 years) (5). Since this cat is so young, he certainly has a long time to develop thyroid tumor growth and hyperthyroidism.

That all said, not all cats with a thyroid mass will become hyperthyroid (6-8), and not all of these thyroid tumors will continue to grow rapidly, become cystic, or transform into a malignancy. However, if this was my cat, I would certainly like to know if the mass is a thyroid adenoma or another type of tumor. In this cat, that should be the first step in managing this cat — we need to know what it is before we can recommend the "best" treatment.

Is the cervical mass of thyroid origin?
If the owner doesn't want to do surgery to remove the mass and submit the tissue for histopathology (the gold standard means of confirming the diagnosis), then the next best test to determine if the mass is of thyroid origin would be thyroid scintigraphy (9-11).

Figure 1: Thyroid scintigraphy in a normal cat

On a normal thyroid scan, only the 2 normal thyroid lobes, salivary glands, and stomach take up the injected radionuclide (see Figure 1). Therefore, if this is not a thyroid tumor (but another type of tumor), it will not be seen on the thyroid scan.

It is also possible that we could have a nonfunctional thyroid tumor that would fail to have any thyroid uptake, but these are very rare, especially in cats. If that were the case, we would only see a single thyroid lobe on the scan— not the 2 normal thyroid lobes, as shown in Figure 1.

Figure 2: Thyroid scintigraphy in a cat with a right thyroid adenoma. 

So the bottom line is this: if the mass takes up radionuclide on thyroid scan (Figure 2), then we know it's a thyroid tumor (most likely an adenoma). If we see two normal thyroid lobes and the mass doesn't take up any of the radionuclide (Figure 1), then the tumor is not of thyroid origin and must be surgically removed to get the answer.

Radioiodine treatment of non-function thyroid tumors
If the thyroid scan does confirm thyroid tumor, and it displays normal to enhanced radionuclide uptake (Figure 2), then the tumor could be treated with radioiodine, even if the cat was not hyperthyroid.

However, if the normal thyroid lobe also takes up some of the injected radionuclide, we may want to consider suppressing the normal thyroid lobe by giving exogenous L-thyroxine for a week or two to shut off pituitary TSH secretion (12). In these cats, I like to repeat the thyroid scan after administration of L-T4 to ensure that the normal lobe no longer takes up the radionuclide. If it does, that means that it will likely take up the administered I-131 and the cat will become hypothyroid.

No treatment with periodic monitoring
Now if the owner does not want to do a thyroid scan (or scintigraphy is not available in your area) and surgical removal is still out, then you have no other option other than to monitor thyroid hormone values and tumor size.

If the mass continues to grow, then surgical or radiation treatment should be strongly considered. If the cat develops hyperthyroidism at some time in the future, then medical treatment could also be considered. However, in such a young cat, I'd still recommend definitive treatment. Who would want to give methimazole to a young cat for up to 15 years or even longer, especially when we know that the thyroid tumor will continue to growth larger over time?

References:

1. Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
2. Peterson M. Hyperthyroidism in cats: What's causing this epidemic of thyroid disease and can we prevent it? J Feline Med Surg 2012;14:804-818. 
3. Hofmeister E, Kippenes H, Mealey KL, et al. Functional cystic thyroid adenoma in a cat. J Am Vet Med Assoc 2001;219:190-193. 
4. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Feline Med Surg 2009;11:116-124. 
5. 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. J Vet Intern Med 2012;26:1523. 
6. Norsworthy GD, Adams VJ, McElhaney MR, et al. Palpable thyroid and parathyroid nodules in asymptomatic cats. J Feline Med Surg 2002;4:145-151. 
7. Ferguson D, Freeman R. Goiter in apparently euthyroid cats In: August JR, ed. Consultations in Feline Internal Medicine. St. Louis: Elsevier Saunders, 2006.
8. Boretti FS, Sieber-Ruckstuhl NS, Gerber B, et al. Thyroid enlargement and its relationship to clinicopathological parameters and T4 status in suspected hyperthyroid cats. J Feline Med Surg 2009;11:286-292. 
9. Daniel GB, Brawnier WR. Thyroid scintigraphy In: Daniel GB, Berry CR, eds. Textbook of Veterinary Nuclear Medicine. 2nd ed. Harrisburg, PA: American College of Veterinary Radiology, 2006;181-199.
10. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. J Vet Intern Med 2012;26:754.
11. Feeney DA, Anderson KL. Nuclear imaging and radiation therapy in canine and feline thyroid disease. Vet Clin North Am Small Anim Pract 2007;37:799-821, viii. 
12. Szumowski P, Rogowski F, Abdelrazek S, et al. Iodine isotope 131-I therapy for toxic nodular goitre: treatment efficacy parameters. Nucl Med Rev Cent East Eur 2012;15:7-13.