[en] 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. Three thyroid function calculation methods were used in 43 patients: 1. thyroid trapping index calculated from the total counts in the neck and thigh images - method A, 2. thyroid trapping index calculated from thyroid region of interest (ROI) counts and ROI transposition over the thigh - method B, 3. standard based percentage thyroid uptake - method C. 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. 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. Copyright (2003) The Australian and New Zealand Society of Nuclear Medicine Inc