Kinetics of the human thyroid trap: a compartmental model.

Thyroidal pertechnetate was measured continuously in normal subjects for 40 min after i.v. injection, using a multicrystal camera. Digital counts (1-min increments) were read directly from the magnetic tape and summed for the thyroid area and for adjacent neck background. The net thyroidal data were used as the basis for development of a compartmental model of the thyroidal trap, using the SAAM program. Input to the trap in the model is plasma pertechnetate radioactivity, measured frequently during the study and fitted to a multiexponential equation. Best fit of the thyroid data was achieved with a model in which the trap is described by two compartments, a fast ("follicular cell") compartment and a slower ("colloid") compartment. Iodide blockade, administered either during the study or 1 hr before its initiation, rapidly blocked the trap at the point of input from plasma into the follicular cells. Iodide did not affect the other parameters of the model.