Validation of a normal range for trapping index in thyroid scintigraphy

Introduction: Thyroid trapping index of 99mTc pertechnetate (neck to thigh ratio) has been used for several decades as a simple alternative to thyroid uptake. Using a trapping index normal range of less than 6, several patients with subjectively diffuse increase in thyroid trapping, biochemical hyperthyroidism and clinical features of thyrotoxicosis have been classified as normal. Methods: Three thyroid function calculation methods were used in 43 patients: thyroid trapping index calculated from the total counts in the neck and thigh images - method A; thyroid trapping index calculated from thyroid ROI counts and ROI transposition over the thigh - method B; standard based percentage thyroid uptake - method C. Results: Five patients (11.6%) were shown to be hyperthyroid by thyroid uptake while trapping index was within normal limits. Correlation amongst all three methods was excellent with methods A and B showing a 0.9870 correlation coefficient, methods A and C a 0.9859 correlation coefficient and methods B and C a 0.9804 correlation coefficient. Conclusion: All 3 methods were shown to provide valid measures of thyroid function. The normal range for trapping index was modified after correlation with the standard thyroid uptake normal limits: Trapping index (total counts) 1.9-5.1; Trapping index (ROI) 3.7-15.1.