Peterson ME, Broome MR.
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. Recently, Harvey et al (Scintigraphic findings in 120 hyper- thyroid cats. JFMS 2009;11: 96) reported that nearly 1 of 5 hyperthyroid cats had multiple areas of increased radionuclide uptake (IRU) visible on thyroid imaging, commonly with intra- thoracic tissue that could not be palpated. Many of those cats, however, were referred for persistent or recurrent hyperthyroidism so this may not represent what is seen in general practice. In this study, we performed thyroid imaging on 917 consecutive hyperthyroid cats that were referred for radioiodine therapy between January and December 2009. Scintigraphy was performed as part of our staging protocol in which thyroid volume is estimated for 131I dose estimation. Of the 917 cats, 594 (65%) had bilateral thyroid lobe uptake in the “expected” neck location, whereas 279 (30%) had unilateral thyroid uptake. The remaining 44 cats (4.8%) had multiple areas of IRU or areas of ectopic tissue not in the neck area. Of the 917 cats, areas of IRU ranged from 1-5 (median 2), with 39 (4.2%) cats having >3 (Table 1). Areas of IRU were located in the neck in 906 (98.8%), thoracic inlet in 105 (11.5%), and in the thorax in 44 (4.8%); 22 cats (2.4%) had IRU in all 3 locations (neck, thoracic inlet, thorax). The estimated size of the thyroid tumor(s) in the 917 cats were small (<2.5 cm3) in 517 (56%), medium-sized (2.5-5 cm3) in 243 (26%), and large (>5 cm3) in 166 (18%). Cats with large thyroid tumors had a higher pretreatment median serum T4 value (20.6 lg/dl) than did cats with medium-sized (13 lg/dl) or small masses (6.4 lg/dl). Cats with large tumor volumes had been hyperthyroid for longer (median, 12 mos) than were cats with small or medium-sized tumors (1 mo). Of the 157 cats with large thyroid masses, 14 were suspected of having thyroid carcinoma, with an overall incidence of 1.7%. Ectopic thyroid tissue was diagnosed in 30 (3.3%) cats; of these, large tumor size was found in only 6 cats (20%). In conclusion, results of this study confirm that multiple areas of hyperfunctional thyroid tissue do develop in hyperthyroid cats, but at an incidence much lower than previously reported. Ectopic thyroid tissue occurs in ~3-4% of hyperthyroid cats, many with only mild to moderate hyperthyroidism. Thyroid carcinoma develops in ~1.5% of cats, but these cats characteristically have huge goiters, very high serum T4 values, and long-standing hyperthyroidism.