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Thyroid scintigraphy in cats with hyperthyroidism

Peterson ME, Broome MR.  Thyroid scintigraphy findings in 2096 cats with hyperthyroidism.  Vet Radiol Ultrasound 2015; 56(1):84-95.  

Thyroid scintigraphy using sodium pertechnate (99mTcO4) is commonly recommended for diagnosis and management of hyperthyroid cats, but this modality is unavailable in many areas as it is only performed in referral facilities, and may add a level of expense to the diagnosis of feline hyperthyroidism that is prohibitive for many owners.  Moreover, very few studies of thyroid scintigraphy of hyperthyroid cats have been published, with the largest including only 135 cats.

This study included 2096 hyperthyroid cats ranging in age from 2 to 23 years, of which only 108 were less than 9 years of age.  All cats were spayed (1138 females) or castrated (958 males) and almost 90% were mixed breed cats; the rest represented a wide variety of popular feline breeds.  Seventy clinically normal and healthy mixed breed control cats (38 spayed females and 32 castrated males), ranging in age from 7 to 16 years, were also included in the study.  These authors routinely perform thyroid scintigraphy in all hyperthyroid cats prior to treatment with radioactive iodine (131I), so almost none of the study cats were referred for persistent or recurrent hyperthyroidism following surgical thyroidectomy.  The study was conducted prospectively from January 2009 to June 2012.  Cats who had been receiving methimazole prior to scintigraphy had to be withdrawn from this drug at least one week prior to admission.

The scintigraphic images of the control cats were analyzed to determine the amount of sodium pertechnate activity in three regions of interest:  (1) thyroid lobes; (2) zygomatic/molar salivary gland; (3) a background area, which in this study was the axillary area. In cats with hyperthyroidism, abnormal scintigraphic findings indicative of thyroid gland disease were classified into 4 groups based on major patterns of scintigraphic intensity:  (1) unilateral disease; (2) bilateral-symmetric in size; (3) bilateral-asymmetric in size; (4) multifocal disease (more than 2 thyroid nodules).

Scintigraphic patterns in some of the patients demonstrated adenomatous ectopic thyroid disease: hyperfunctional thyroid tissue that was not in its normal mid-ventral cervical location.  Ectopic thyroid tissue, if present, is usually found in the lingual/sublingual (about 10% of study cats with ectopic thyroid tissue) or cranial thoracic regions (over 80% of the study cats with ectopic thyroid tissue), and its presence can be explained by incomplete descent of the thyroglossal duct in the embryo or excessive migration of thyroid tissue. Ectopic thyroid tissue is subject to the same pathological processes—inflammation, hyperplasia, and tumorigenesis—that can develop in eutopic thyroid tissue.  Suspected thyroid carcinoma was identified in some of the cats as a massive volume of hyperfunctional thyroid tissue, with multiple, extensive areas of increased 99mTcO4 uptake, extension of tumor through the thoracic inlet into the thorax, and/or metastasis to regional lymph nodes or lung.

In the 2096 hyperthyroid cats, the median value for the ratio of intensity of uptake of radionuclide by the thyroid tissue to that of salivary gland tissue (T/S ratio) was 6.9, significantly higher than the T/S ratio of 0.9 in the clinically normal control cats.  The ratio of median value of intensity of uptake of radionuclide by the thyroid tissue to that in the background area (T/B) was also significantly higher at 25.1 in the hyperthyroid cats than in the control cats (3.0).  There also was a significant correlation between the pre-treatment serum T4 concentration and both the T/S and T/B ratios in the hyperthyroid cats.  Of the 2096 hyperthyroid cats, 98.7% had a high T/S ratio (>1.5), and 96.1% had a high T/B ratio (>6.1); 191 (9.1%) of the cats had serum T4 within reference range limits (0.8-4.0 micrograms/dL), although they had clinical and physical examination findings such as a palpable thyroid nodule as well as a high serum free T4 concentration.  The sensitivity of both the T/S and T/B ratios were significantly higher as diagnostic tests for hyperthyroidism in this population of cats than the basal T4 concentrations.

Four major patterns of thyroid disease based on scintigraphic findings in the cats:  665 (31.7%) had unilateral disease, 1060 (50.6%) had bilateral-asymmetric disease (the two thyroid lobes were unequal in size), 257 (12.3%) had bilateral-symmetric disease (both thyroid lobes were similar in size), and 81 (3.9%) had multifocal disease (> 3 areas of increased radionuclide uptake).  All cats with multifocal disease either had ectopic thyroid nodules (n = 51) or thyroid carcinoma (n = 30).  Only 33 cats (1.6%) had one or two areas of increased pertechnate uptake that did not fit into any of the four previously described major patterns of classification.  All of the cats with multifocal disease (n = 81) and the 33 that were unclassified had areas of radionuclide uptake that were outside the area of the normal cervical location of the thyroid gland.  Intrathoracic thyroid  masses can be distinguished from ectopic mediastinal thyroid masses on the basis of their left- or right-lateral location; ectopic thyroid tissue is typically found medially on the ventral midline, where the thyroglossal duct descended in the embryo. Only about 5% of the cases studied had extracervical thyroid disease within the thoracic cavity.

In the cats with suspected thyroid carcinoma, the diagnosis was not confirmed with a thyroid biopsy and histopathology due to the risk of general anesthesia in this population with uncontrolled hyperthyroidism.  Almost all cats with suspected thyroid carcinoma can be cured with higher doses of radioiodine and usually have a good prognosis, in contrast to dogs with a functional thyroid carcinoma, whose prognosis is generally guarded to poor.  [PJS]

See also:
Peterson ME. Diagnostic testing for hyperthyroidism in cats:  more than just T4.  J Feline Med Surg 2013;15:765-77.