The elimination of errors caused by prior technetium-99m scintigraphy on iodine-131 thyroid uptake measurements

Coincidence summing of technetium-99m (99mTc) pulses may disturb iodine-131 (131I) thyroid uptake measurements if these are obtained after 99mTc thyroid scintigraphy performed on the same day. The magnitude of the error was assessed in 26 patients. It was found that when pre-dose measurements were subtracted, the 6-hour and 24-hour 131I uptakes could be falsely decreased by up to 137 percentage points using a lower discriminator level of 250 keV and by up to 35 percentage points with a discriminator level of 300 keV. When pre-dose measurements from 99mTc were ignored, there was a general increase in 131I uptake values at 6 hours. Increases of up to 22 and 9 percentage points occurred when 250 keV and 300 keV lower discriminator levels respectively were used. These errors may be eliminated by performing 131I uptake measurements through a 1 mm lead filter.