QUANTITATIVE PERTECHNETATE THYROID SCINTIGRAPHY AND THE ULTRASONOGRAPHIC APPEARANCE OF THE THYROID GLAND IN CLINICALLY NORMAL HORSES

We characterized the scintigraphic and sonographic appearance of the thyroid gland in clinically normal horses to establish the value of these modalities for assessment of the thyroid gland in this species. Horses were divided into two age groups. One group consisted of eight horses between 3 and 10 years of age and the other of seven horses between 11 and 20 years of age. Total T4 concentrations were within the laboratory reference interval in all horses. Thyroid to salivary (T/S) ratio, percent dose uptake of pertechnetate (Na99mTcO4) and thyroid lobe volume were calculated. The echogenicity of thyroid lobes and presence of nodules was documented. The two groups were compared using appropriate parametric and nonparametric statistics. Mean total T4 concentration was lower in older horses. Sixty minute mean +/- standard deviation (SD) T/S ratios for old vs. young horses were 5.8 +/- 3.0 and 5.3 +/- 2.2, respectively. Sixty-minute median and interquartile ranges for percent dose uptake of pertechnetate for old vs. young horses were 3.64% (1.5-3.98%) and 2.55% (2.33-2.90%), respectively. Mean +/- SD thyroid lobe volume for old vs. young horses were 18.93 +/- 5.16 cm 3 and 13.55 +/- 3.56 cm3, respectively. Differences between groups were not significant. Most thyroid lobes were hyper or isoechoic to the sternocephalicus muscle. Prevalence of thyroid nodules did not differ between groups. Further study is needed to determine if thyroidal percent dose uptake is significantly different in horses with thyroid dysfunction and if it is clinically useful.