Quantitative differences between the thyroid uptake of 123I and 99mTc.

Unlike 123I, sodium pertechnetate Tc99m is known to be trapped by the thyroid gland without being further processed. Whether this property alone explains the total difference between the early thyroid uptake of these two isotopes was studied in a group of goitrous patients. The absolute activities of 99mTc were measured by a conjugate-view counting method and those of 123I by a coincidence counting method. The uptake was continuously registered to give uptake curves which were analyzed according to a three-compartment model of the thyroid. Both isotopes are postulated to bind to some carriers and then to be transported into the thyroid gland before they are further metabolized. Although the differences observed between the two uptakes were somewhat model dependent, the application of compartmental analysis provides a more detailed description of the differences between the two uptakes which occur even before the organification step.