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

Abstract

Our aim was to investigate thyroid:thyroid (T:T) ratio and visual inspection for assessing thyroid-lobe asymmetry in suspected hyperthyroid cats. Although thyroid-salivary asymmetry is a preferred test, inherent thyroid symmetry may assist image interpretation. Association was determined using a scatter plot and Spearman's rank correlation. Agreement was assessed using the kappa (K) statistic. Accuracy was assessed by sensitivity and specificity. Hyperthyroidism was diagnosed in 33/48 (69%) cats based on elevated serum total thyroxine level. Using two Wilcoxan rank-sum tests, a significant difference (P < 0.0001) was detected between cats with and without hyperthyroidism for both methods of assessing thyroid symmetry. For the 18 cats with T:T ratios < or = 1.5, there was poor correlation between the two methods (r(s) = 0.39). Using a cut-point of 1.5 for the T:T ratio, the test accurately predicted hyperthyroidism in 28/33 cats (sensitivity, 85%; 95% confidence interval (CI), 71-99%) and correctly predicted that hyperthyroidism was absent in 14/15 cats (specificity, 93%; CI, 77-100%). For visual inspection, agreement for diagnosing hyperthyroidism was excellent between methods (kappa = 0.82), within the same examiner (weighted kappa = 0.85) and between examiners (weighted kappa = 0.89). Considering cats with only definitely asymmetric thyroid lobes as positive, visual inspection accurately predicted hyperthyroidism in 28/33 cats (sensitivity, 85%; CI, 71-99%) and correctly predicted that hyperthyroidism was absent in 11/15 cats (specificity, 73%; CI, 48-99%). Thyroid-lobe asymmetry occurs more frequently in hyperthyroid than in euthyroid cats but caution should be exercised because some euthyroid cats have asymmetric thyroid glands.

Quantitative Thyroid Scintigraphy in Greyhounds Suspected of Primary Hypothyroidism

“…Uptake of 99m TcO 4 − by the thyroid in normal dogs is described as symmetric 13,15,16,27 . Asymmetric thyroid uptake of 99m TcO 4 − has been found in animals with thyroid neoplasia or hyperthyroidism and in normal cats 28 . The reason for the asymmetry noted in the present study is unclear but it likely represents a normal feature of greyhounds.…”

“…13,15,16,27 Asymmetric thyroid uptake of 99m TcO 4 À has been found in animals with thyroid neoplasia or hyperthyroidism and in normal cats. 28 The reason for the asymmetry noted in the present study is unclear but it likely represents a normal feature of greyhounds. A difference in thyroid lobe volume might be the cause of the asymmetry.…”

Quantitative Pertechnetate Thyroid Scintigraphy and the Ultrasonographic Appearance of the Thyroid Gland in Clinically Normal Horses

“…66 Recently it has been reported that at least some euthyroid cats have asymmetric thyroid lobes during visual inspection of pertechnetate scintigrams. 68 Margins of the thyroid lobes should be smooth and regular. Ectopic thyroid tissue is not usually seen in normal cats.…”

Hyperthyreoïdie bij katten: Deel II: scintigrafische diagnose en radiojoodbehandeling

“…A normal thyroid scintigram in cats shows the two thyroid lobes as elongated, oval structures in the cervical region, smoothly delineated, symmetrical in size and position and with a homogeneous distribution of the radionuclide (Figure 2). A certain level of thyroid lobe asymmetry can be present in euthyroid cats (Scrivani et al, 2007). Ectopic thyroid tissue is less commonly seen.…”

“…In the literature, the potential role of the thyroid:thyroid (T:T) ratio in the diagnosis of hyperthyroidism has been reported only once (Scrivani et al, 2007). The T:T ratio refers to a lobe-to-lobe ratio, where the value of the most active thyroid lobe is divided by the value of the less active lobe.…”