Thyroid Scintigraphy Findings in 2096 Cats With Hyperthyroidism

Abstract

Thyroid scintigraphy is currently the reference standard for diagnosing and staging cats with hyperthyroidism, but few studies describing the scintigraphic characteristics in a large number of cats have been reported. The objective of this study was to better characterize thyroid scintigraphy findings by evaluating 2096 consecutive cats with hyperthyroidism that were referred over a 3.5-year period. Of these cats, 2068 (98.7%) had a high thyroid-to-salivary ratio (>1.5), whereas 2014 (96.1%) were found to have a high thyroid-to-background ratio (>6.1). When the patterns of the cats' thyroid disease were recorded, 665 (31.7%) had unilateral disease, 1060 (50.6%) had bilateral-asymmetric disease (two thyroid lobes unequal in size), 257 (12.3%) had bilateral-symmetric disease (both lobes similar in size), and 81 (3.9%) had multifocal disease (≥3 areas of increased radionuclide uptake). The number of areas of (99m) TcO(-) 4 uptake in the 2096 cats ranged from 1 to 6 (median, 2), located in the cervical area in 2057 (98.1%), thoracic inlet in 282 (13.5%), and in the thoracic cavity in 115 (5.5%). Ectopic thyroid tissue (e.g. lingual or mediastinal) was diagnosed in 81 (3.9%) cats, whereas thyroid carcinoma was suspected in 35 (1.7%) of the cats. The results of this study support conclusions that most hyperthyroid cats have unilateral or bilateral thyroid nodules, but that multifocal disease will develop in a few cats that have ectopic thyroid disease or thyroid carcinoma. Both ectopic thyroid disease and thyroid carcinoma are relatively uncommon in hyperthyroid cats, with a respective prevalence of ∼4% and ∼2% in this study.

ANIMAL MODELS OF DISEASE: Feline hyperthyroidism: an animal model for toxic nodular goiter

“…Calculation of an increased value for thyroidto-salivary ratio also provide a sensitive means of diagnosing hyperthyroidism in cats (Peterson & Broome 2014b), as has been reported in man (Sostre & Parikh 1979, Anjos et al 2006, Kandeel et al 2009). …”

“…mongrel cats). Female cats also develop hyperthyroidism more commonly than males (1.25:1; Peterson et al 1988, Peterson & Becker 1995, Peterson & Broome 2014b, although the female sex predilection is not as marked as in human patients with toxic nodular goiter (R4:1; Vanderpump 2011, Paschke 2013.…”

“…However, cats with subclinical hyperthyroidism have normal serum T 4 and T 3 values (Wakeling et al 2007(Wakeling et al , 2011, and up to half of cats with early or mild hyperthyroidism have serum T 4 within the reference interval limits (Peterson 2001. In these cats, thyroid imaging or T 3 suppression testing can be performed to aid in early diagnosis (Peterson et al 1990, Peterson 2001, Peterson & Broome 2014b). However, serum total T 4 and T 3 concentrations will eventually increase into the thyrotoxic range upon retesting a few weeks to months later as the goiter size continues to increase.…”

Evaluation of Serum Thyroid‐Stimulating Hormone Concentration as a Diagnostic Test for Hyperthyroidism in Cats

“…In all methimazole‐treated cats, the drug was discontinued ≥1 week before evaluation. All of these cats had scintigraphic evidence of hyperthyroidism, with unilateral, bilateral, or multifocal nodular enlargement (ie, “hot” thyroid nodules) and lack of radionuclide uptake into normal thyroid tissue 9. Additionally, all cats had high thyroid‐to‐salivary gland (T/S) ratios, high thyroid‐to‐background (T/B) ratios, or both 9…”

“…Tests of the hypothalamic‐pituitary‐thyroid (HPT) axis (eg, T 3 suppression test) can help in diagnosis of occult hyperthyroidism but suffer from problems with owner compliance and equivocal test results 3, 6, 8. Consequently, quantitative thyroid scintigraphy, with calculation of thyroid‐to‐salivary and thyroid‐to‐background ratios, is considered the most accurate test for occult hyperthyroidism in cats 9. Unfortunately, few veterinarians have access to scintigraphic equipment, and simpler ways of diagnosing hyperthyroidism are needed in cats with occult disease.…”