Is Fine-Needle Aspiration Helpful in Diagnosing Thyroid Carcinomas in Cats?

Huge thyroid mass (goiter) in cat with long-standing hyperthyroidism

I have a 15-year old, DSH, male hyperthyroid cat that can no longer be controlled adequately with methimazole, despite an increase in the dose to 10 mg twice daily. This cat was diagnosed 4 years ago and now has a very large palpable goiter (see photos above).

I'm considering the use of ultrasound-guided fine-needle aspiration (FNA) to get a small biopsy of the enlarged thyroid tumor to determine whether this is cancer (thyroid carcinoma) or just a very large benign thyroid mass (adenoma or adenomatous hyperplasia).

Is it possible to determine if the tumor is malignant or not with FNA of the affected thyroid? Or do I need a tissue biopsy or thyroid scan to diagnose thyroid carcinoma?

My Response:

Unfortunately, FNA of the enlarged goiter is not that helpful in diagnosis of thyroid carcinoma in cats. Although use of FNA is commonly used as part of the workup of human patients with thyroid nodules (1,2) these human patients are generally not suffering from hyperthyroidism, so the pathological features of the thyroid nodule is much different from the nodules we see in hyperthyroid cats. In addition, even if human patients, false-negative results of FNA are not uncommon (3-5).

Endocrine pathology can be very difficult (6,7), and in many cats it can be difficult to determine adenoma from carcinoma even if the whole thyroid tumor has been removed and routine histopathology is performed.

To complicate thyroid pathology even more, I believe that feline thyroid carcinoma can arise from transformation of benign thyroid adenoma/adenomatous hyperplasia to thyroid carcinoma (see references below.  In support of this, investigators have recently reported that some hyperthyroid cats will have areas of adenoma adjacent to areas of carcinoma within the same thyroid lobe on biopsy (8).

In addition, my own studies have shown that the prevalence of thyroid carcinoma in hyperthyroid cats receiving long-term methimazole treatment increases considerably over time, rising to approximately 20% in cats treated for over 4 years (9-11). This suggests that, at least in some cats with long-standing hyperthyroidism, there may be transformation of thyroid adenomatous hyperplasia/adenoma to thyroid carcinoma (8). If that is the case, the pathogenesis of thyroid adenoma and thyroid carcinoma may be identical in nature, rather than these representing two separate tumor processes.

Therefore, to make a diagnosis of thyroid carcinoma, the best diagnostic approach would be surgical tissue biopsy. Even if that is done, however, the pathologist must look at multiple sections of the thyroid tissue in order to rule out concurrent thyroid adenoma and carcinoma within the removed thyroid lobe.

Thyroid scintigraphy can be very helpful in evaluating these cats with large thyroid masses (8-11). Although a definitive diagnosis can be made only with thyroid biopsy, the scintigraphic findings of multiple areas of thyroid uptake, irregular tumor borders, and soft tissue invasion (especially into the thoracic cavity) would all suggest the presence of thyroid carcinoma (8,11).

References:

1. Chowdhury S, Mukherjee S, Mukhopadhyay S, et al. The thyroid nodule--evaluation and management. J Indian Med Assoc 2006;104:568-570, 572-563. 
2. Langer JE, Baloch ZW, McGrath C, et al. Thyroid nodule fine-needle aspiration. Semin Ultrasound CT MR 2012;33:158-165. 
3. Samir AE, Vij A, Seale MK, et al. Ultrasound-guided percutaneous thyroid nodule core biopsy: clinical utility in patients with prior nondiagnostic fine-needle aspirate. Thyroid 2012;22:461-467. 
4. Grani G, Calvanese A, Carbotta G, et al. Intrinsic factors affecting adequacy of thyroid nodule fine-needle aspiration cytology. Clin Endocrinol (Oxf) 2013;78:141-144. 
5. Mehanna R, Murphy M, McCarthy J, et al. False negatives in thyroid cytology: Impact of large nodule size and follicular variant of papillary carcinoma. Laryngoscope 2013 (in press). 
6. Carpenter JL, Andrews LK, Holzworth J. Tumors and tumor-like lesions In: Holzworth J, ed. Diseases of the Cat: Medicine and Surgery. Philadelphia: W.B. Saunders, 1987;406-596.
7. Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Vet Clin North Am Small Anim Pract 1994;24:541-565.  
8. 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.  
9. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 ACVIM Forum; 2011 June 15–19; Denver, CO. American College of Veterinary Internal Medicine, pp 104–106.
10. 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. 
11. Peterson ME and Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. J Vet Intern Med 2012; 26: 54. 
12. 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.