Accuracy of Increased Thyroid Activity During Pertechnetate Scintigraphy by Subcutaneous Injection for Diagnosing Hyperthyroidism in Cats

Abstract

Our purpose was to determine the accuracy of increased thyroid activity for diagnosing hyperthyroidism in cats suspected of having that disease during pertechnetate scintigraphy using subcutaneous rather than intravenous radioisotope administration. Increased thyroid activity was determined by two methods: the thyroid:salivary ratio (T:S) and visual inspection. These assessments were made on the ventral scintigram of the head and neck. Scintigraphy was performed by injecting sodium pertechnetate (111 MBq, SQ) in the right-dorsal-lumbar region; static-acquisition images were obtained 20 min after injection. We used 49 cats; 34 (69%) had hyperthyroidism based on serum-chemistry analysis. Using a Wilcoxon's rank-sum test, a significant difference (P < 0.0001) was detected in the T:S between cats with and without hyperthyroidism. Using a decision criterion of 2.0 for the T:S, the test accurately predicted hyperthyroidism in 32/34 cats (sensitivity, 94%; 95% confidence interval (CI), 85-100%) and correctly predicted that hyperthyroidism was absent in 15/15 cats (specificity, 100%; CI, 97-100%). Using visual inspection, the test accurately predicted hyperthyroidism in 34/34 cats (sensitivity, 100%; CI, 99-100%) and correctly predicted that hyperthyroidism was absent in 12/15 cats (specificity, 80%; CI, 56-100%). The positive and negative predictive values were high for a wide range of prevalence of hyperthyroidism. And, the test had excellent agreement within and between examiners. Therefore, detecting increased thyroid activity during pertechnetate scintigraphy by subcutaneous injection is an accurate and reproducible test for feline hyperthyroidism.

Feline Hyperthyroidism

“…One report described successful thyroid scintigraphy in cats after SC administration of the isotope; however, a direct comparison between IV and SC administration was not made (Page et al, 2006). One report described successful thyroid scintigraphy in cats after SC administration of the isotope; however, a direct comparison between IV and SC administration was not made (Page et al, 2006).…”

Investigation of Two Methods for Assessing Thyroid‐lobe Asymmetry During Pertechnetate Scintigraphy in Suspected Hyperthyroid Cats

“…The order of patients was randomized for each of the trials, using an online number generator based on atmospheric noise. During the first review, completed over two sittings, regions of interest were drawn around both thyroid lobes and both salivary glands as previously described 1 . The counts/pixel for each region was recorded.…”

“…The study population consisted of 48 adult (≥4‐year‐old) cats admitted prospectively to the Cornell University Hospital for Animals (CUHA) between January 2004 and April 2005 for suspected hyperthyroidism based on signalment, history, and clinical findings. The inclusion and exclusion criteria, demographic data, and procedures performed were previously reported, except the scintigram from 1 cat was lost 1 . Hyperthyroidism was diagnosed in 33/48 (69%) cats based on serum‐chemistry analysis.…”

“…To assess inter‐examiner agreement for visual inspection of thyroid‐lobe asymmetry, the results of the second review (for the first examiner) were compared with the results of the second examiner using the weighted κ statistic. Intraexaminer agreement for the T:T ratio was not assessed because we already reported good agreement when using regions of interest, at least for the larger thyroid lobe and ipsilateral salivary gland 1 …”

“…W e previously reported some false‐positive results for hyperthyroidism during visual inspection of pertechnetate scintigrams 1 . One possible reason for this decreased specificity was bias from observing asymmetry of the thyroid lobes vs. only assessing differential uptake in the thyroid and salivary glands.…”