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.
M. E. Peterson and M. R. Broome. Animal Endocrine Clinic, New York, NY and Advanced Veterinary Medical Imaging, Tustin, CA.
Thyroid scintigraphy provides valuable information regarding both thyroid anatomy and physiology and plays an integral role in the diagnosis, staging, and management of feline thyroid disease. In this study, we performed thyroid imaging on 1,572 consecutive hyperthyroid cats that were referred for radioiodine therapy between January 2009 and December 2011. Scintigraphy was performed as part of our staging protocol in which thyroid volume is estimated for 131-I dose estimation (as described in Vet Radiol Ultrasound 1996;27:141). In each cat, the location of each area of increased radionuclide uptake (IRU) was also recorded (cervical, thoracic inlet, chest). Finally, each scan was evaluated for features suggesting malignancy (multiple, extensive areas of IRU, heterogeneous pattern of IRU with irregular, spiculated margins, extension of tumor through thoracic inlet into the thorax, and metastasis to regional lymph nodes or lung).
Of the 1,572 cats, most had been recently diagnosed. In 228 cats, however, the interval between diagnosis and 131I treatment ranged from >1 to 6.1 years; almost all of these cats had received long-term antithyroid drug treatment. The 1,572 cats were divided into 5 groups based on interval from diagnosis to 131I treatment: Group 1 (0-1 year), 1,344 cats; Group 2 (>1-2 years), 114 cats; Group 3 (>2-3 years), 62 cats; Group 4 (>3-4 years), 29 cats; and Group 5 (>4-6.1 years), 23 cats.
When the estimated thyroid volumes in the 5 groups of cats were compared, a progressive, significant (P< 0.001) increase in median tumor volume occurred: 2.1 cm3 (Group 1); 4.1 cm3 (Group 2); 5.7 cm3 (Group 3); 6.1 cm3 (Group 4); and 7.7 cm3 (Group 5). The prevalence of cats with areas of IRU within the thoracic cavity also increased progressively: 5.1% (Group 1); 7.9% (Group 2); 14.5% (Group 3); 17.2% (Group 4); and 26.1% (Group 5). Finally, the prevalence of suspected thyroid carcinoma (25 of the 1,572 cats) also increased progressively: 0.7% (Group 1); 3.5% (Group 2); 6.5% (Group 3); 10.4% (Group 4); and 21.7% (Group 5) (see Figure below). In contrast, no increase in prevalence of ectopic thyroid tissue was found: 2.9% (Group 1); 2.8% (Group 2); 3.2% (Group 3); 6.9% (Group 4); and 4.3% (Group 5). In conclusion, our results indicate that hyperfunctional thyroid tissue continues to grow and enlarge over time. Thyroid carcinoma is extremely rare in cats with recently diagnosed hyperthyroidism, but the prevalence increases dramatically over time, suggesting that transformation from benign disease is common in cats controlled medically.
