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Use of I-123 in Early Radioiodide Uptake and Its Suppression in Children and Adolescents with Hyperthyroidism

UCLA School of Medicine, Los Angeles, California

Correspondence: For reprints contact: Wai-Nang P. Lee, Div. of Endocrinology, Dept. of Pediatrics, UCLA School of Medicine, Los Angeles, CA 90024.

ABSTRACT

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 µg/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 yr. Only two patients with nonsuppressible uptake achieved remission from hyperthyroidism, perhaps because of coexistence of thyroiditis.