Use of I-123 in early radioiodide uptake and its suppression in children and adolescents with hyperthyroidism.

Absolute activity measurement of I-123 by coincidence counting was used to study the early thyroidal iodide uptake in 20 hyperthyroid children. Patients were pretreated either with methimazole or propylthiouracil before injection of Na123I. The usual method of analysis of the early uptake was modified to account for a rapidly equilibrating compartment, to give thyroidal iodide trapping rate constant (K1) and absolute iodide uptake (AIU). The suppressibility of the early uptake by triiodothyronine (T3) was evaluated in some patients. The upper limit of normal for K1 was 0.03 min-1 and for AIU was 0.04 microgram/min. In the hyperthyroid subjects, K1 and AIU were in the hyperthyroid range before and after T3 suppression. For patients with suppressible uptake, remission from hyperthyroidism was maintained for 6 mo to 2 1/2 yr. Only two patients with nonsuppressible uptake achieved remission from hyperthyroidism, perhaps because of coexistence of thyroiditis.