INVESTIGATION OF TWO METHODS FOR ASSESSING THYROID‐LOBE ASYMMETRY DURING PERTECHNETATE SCINTIGRAPHY IN SUSPECTED HYPERTHYROID CATS

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 (κ) 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 ≤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 (κ=0.82), within the same examiner (weighted κ=0.85) and between examiners (weighted κ=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.