Hyperthyroidism is a common disease in cats. Approximately 70% of cats with hyperthyroidism have bilateral thyroid gland enlargement.1–3 Hyperthyroidism caused by thyroid gland neoplasia develops in only 1% to 2% of cats.1,4
In cats, the thyroid gland is bilobed and located lateral to the trachea.5 The thyroid gland consists of separate masses (lobes) that are not connected by an isthmus5,6; in humans, the thyroid gland has an isthmus.7
In humans, normal thyroid tissue is hyperattenuating to surrounding tissues by use of precontrast CT.7–9 The high attenuation of normal thyroid tissue is directly associated with its iodine concentration.9–12 In clinically normal cats, thyroid tissue is hyperattenuating to surrounding soft tissues with precontrast CT numbers of 123.2 Hounsfield units (95% confidence interval, 119.4 to 127.1 Hounsfield units).13
In humans, volume determination of thyroid glands is easy, reliable, and repeatable.14 Compared with ultrasonography, CT volume determination of large thyroid glands caused by goiter in 27 people was more reproducible.15 Another study16 estimated normal thyroid gland size by use of the thyroid gland-to-trachea ratio. Use of the thyroid gland-to-trachea ratio was easier than calculating the volume of the entire thyroid gland. In clinically normal cats, the size and volume of thyroid glands have been determined by use of ultrasonography.17 Ultrasonographically, thyroid glands in hyperthyroid cats were larger than thyroid glands in clinically normal cats.17
Ablation of hyperactive thyroid tissue in cats by use of iodine 131 (131I) is common. However, the optimal method of 131I dose determination is controversial.1 Methods include measuring the kinetics of 131I uptake by the thyroid gland18; estimating volume on the basis of thyroid scintigraphy18,19; use of a combination of clinical signs, thyroid gland size, and magnitude of serum thyroxine concentration20; and use of an empirical dose of 148 to 185 MBq (4 to 5 mCi) of 131I.21,22 An accurate method for determining the volume of thyroid tissue in cats may help eliminate some of the controversy.
Use of CT is increasing in veterinary medicine. Knowing the dimensions of thyroid glands in clinically normal cats should aide in the evaluation of the cervical region. The purpose of the study reported here was to determine the dimensions and volume of thyroid tissue in clinically normal cats by use of CT.
Milligrams of iodine
Technetium Tc 99m
Mattoon JS, Drost WT, Samii VF, et al. Helical computed tomography of normal feline pancreas (abstr). Vet Radiol Ultrasound 2003;44:106.
Mattoon JS, Drost WT, Samii VF, et al. Helical computed tomography of the normal feline lung (abstr). Vet Radiol Ultrasound 2003;44:103.
Picker PQS, Philips Medical Systems NA, Bothell, Wash.
Omnipaque, Nycomed Inc, Princeton, NJ.
Peterson ME. Hyperthyroidism. In: Ettinger SJ, Feldman EC, eds. Textbook of veterinary internal medicine: diseases of the dog and cat. 5th ed. Philadelphia: WB Saunders Co, 2000;1400–1419.
Peterson ME, Kintzer PP, Cavanagh PG, et al.Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. J Am Vet Med Assoc 1983;183:103–110.
Peterson ME, Becker DV. Radionuclide thyroid imaging in 135 cats with hyperthyroidism. Vet Radiol 1984;25:23–27.
Turrel JM, Feldman EC, Nelson RW, et al.Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981–1986). J Am Vet Med Assoc 1988;193:359–364.
Waters DJ. Endocrine system. In: Hudson LC, Hamilton WP, eds. Atlas of feline anatomy for veterinarians. Philadelphia: WB Saunders Co, 1993;127–134.
Dyce KM, Sack WO, Wensing CJG. The endocrine glands. In: Dyce KM, Sack WO, Wensing CJG, eds. Textbook of veterinary anatomy. Philadelphia: WB Saunders Co, 1987;205–211.
Weber AL, Randolph G, Aksoy FG. The thyroid and parathyroid glands. CT and MR imaging and correlation with pathology and clinical findings. Radiol Clin North Am 2000;38:1105–1129.
Weissman JL, Curtin HD, Johnson JT. Thyroid gland after total laryngectomy: CT appearance. Radiology 1998;207:405–409.
Sekiya T, Tada S, Kawakami K, et al.Clinical application of computed tomography to thyroid disease. Comput Tomogr 1979;3:185–193.
Arger PH, Jennings AS, Gordon LF, et al.Computed tomography findings in clinically normal and abnormal thyroid patients. J Comput Tomogr 1985;9:111–117.
Imanishi Y, Ehara N, Mori J, et al.Measurement of thyroid iodine by CT. J Comput Assist Tomogr 1991;15:287–290.
Imanishi Y, Ehara N, Shinagawa T, et al.Correlation of CT values, iodine concentration, and histological changes in the thyroid. J Comput Assist Tomogr 2000;24:322–326.
Drost WT, Mattoon JS, Samii VF, et al.Computed tomographic densitometry of normal feline thyroid glands. Vet Radiol Ultrasound 2004;45:112–116.
Hermans R, Bouillon R, Laga K, et al.Estimation of thyroid gland volume by spiral computed tomography. Eur Radiol 1997;7:214–216.
Nygaard B, Nygaard T, Court-Payen M, et al.Thyroid volume measured by ultrasonography and CT. Acta Radiol 2002;43:269–274.
Prince JS, Stark P. Normal cross-sectional dimensions of the thyroid gland on routine chest CT scans. J Comput Assist Tomogr 2002;26:346–348.
Wisner ER, Theon AP, Nyland TG, et al.Ultrasonographic examination of the thyroid gland of hyperthyroid cats: comparison to (TcO4 −)-Tc-99m scintigraphy. Vet Radiol Ultrasound 1994;35:53–58.
Broome MR, Turrel JM, Hays MT. Predictive value of tracer studies for 131I treatment in hyperthyroid cats. Am J Vet Res 1988;49:193–197.
Forrest LJ, Baty CJ, Metcalf MR, et al.Feline hyperthyroidism: efficacy of treatment using volumetric analysis for radioiodine dose calculation. Vet Radiol Ultrasound 1996;37:141–145.
Adams WH. Thyroid radiotherapy: iodine-131. In: Berry CR, Daniel GB, eds. Handbook of veterinary nuclear medicine. Raleigh, NC: North Carolina State University, 1996;80–84.
Chun R, Garrett LD, Sargeant J, et al.Predictors of response to radioiodine therapy in hyperthyroid cats. Vet Radiol Ultrasound 2002;43:587–591.
Drost WT, McLoughlin MA, Mattoon JS, et al.Determination of extrarenal plasma clearance and hepatic uptake of technetium-99m-mercaptoacetyltriglycine in cats. Am J Vet Res 2003;64:1076–1080.
Breiman RS, Beck JW, Korobkin M, et al.Volume determinations using computed tomography. AJR Am J Roentgenol 1982;138:329–333.
Weichselbaum RC, Feeney DA, Jessen CR. Evaluation of relationships between pretreatment patient variables and duration of isolation for radioiodine-treated hyperthyroid cats. Am J Vet Res 2003;64:425–427.
Nygaard B, Nygaard T, Jensen LI, et al.Iohexol: effects on uptake of radioactive iodine in the thyroid and on thyroid function. Acad Radiol 1998;5:409–414.
Yu MD, Shaw SM. Potential interference of agents on radio-iodide thyroid uptake in the euthyroid rat. J Nucl Med 2003;44:832–838.
Lerman LO, Bentley MD, Bell MR, et al.The effect of a low-osmolar radiographic contrast medium on in vivo and postmortem renal size. Invest Radiol 1991;26:992–997.
Silverman PM, Newman GE, Korobkin M, et al.Computed tomography in the evaluation of thyroid disease. AJR Am J Roentgenol 1984;142:897–902.