Thyroid uptake measurements: the influence of gland depth, gland mass and lobe separation.

Phantom measurements were carried out to define the dependence of standard thyroid uptake measurements on gland depth, gland mass and lobe separation. In the case of 99Tcm and 131I, depth can cause a variation of up to a factor of two in the apparent uptake, while in the case of 125I the variation due to depth is a factor of about four. By contrast, gland mass and lobe separation cause only small errors, of the order of 2 and 10% respectively for medium energy isotopes. These errors are particularly important when the uptake value is used to guide prescription of 131I for therapy of thyrotoxicosis.