Marsolais ME, Mott J, Berry CR: Diagnosis of feline hyperthyroidism using thyroid scintigraphy. J Vet Intern Med 17: 393, 2003.

Diagnosis of hyperthyroidism is traditionally made by demonstrating elevations in the serum thyroxine (T4) level; however, fluctuations in T4 levels and the possibility that concurrent disease can suppress thyroid hormone concentrations often limit the diagnostic accuracy of this test. Hyperthyroidism has been definitively diagnosed by demonstrating a thyroid to salivary gland (T:S) ratio greater than 1:1 using sodium 99mTc-pertechnetate thyroid scintigraphy. The purpose of this study was to evaluate cats with a palpable thyroid nodule and normal serum thyroxine levels (T4 between 1.0-4.0 mcg/dL) for the presence of hyper-functional thyroid tissue based on results of sodium 99mTc-pertechnetate thyroid scintigraphy.

This study evaluated 23 cats with a palpable thyroid nodule and normal T4 values. Free T4 by equilibrium dialysis [fT4(ED)] was measured in 16/23 cats. Nuclear scintigraphy was performed on all cats using standard views and imaging parameters. These results were compared with established normal T:S ratios to identify hyperthyroid cats. Cats positive on the basis of scintigraphy (T:S ratio >1:1) were further classified as unilateral, bilateral, or ectopic disease. Data was analyzed using single analysis of variance comparing average T4, average fT4(ED), and prevalence of clinical signs and physical exam findings between the positive and negative scintigraphy groups. A student's t-test was used to demonstrate statistical difference in T:S ratios between positive and negative groups. A p value of 0.05 was determined significant.

Seventeen of 23 cats (74%) were diagnosed to be hyperthyroid based on thyroid scintigraphy, while the remaining 6/23 were negative. There was no significant difference in serum thyroxine levels between cats in the positive and negative groups. A significant difference (p<0.0006) was noted between the groups as measured by fT4(ED). Positive cats had an average ± SD fT4(ED) of 82 ± 14 pmol/L compared with negative cats having an average fT4(ED) of 44 ± 18 pmol/L. As expected, cats considered positive had a significantly higher T:S ratio when compared to negative cats (1.98 ± 0.6 vs. 0.87 ± 0.2, p<0.001). Negative cats had a significantly higher prevalence of a gallop rhythm (p< 0.000015) and weight loss (p<0.038) when compared with positive cats.

This study demonstrates that a palpable thyroid nodule is suggestive of feline hyperthyroidism despite normal serum thyroxine levels. In these cats additional diagnostics such as thyroid scintigraphy or measurement of fT4(ED) is recommended for the diagnosis of hyperthyroidism.