Thyroid uptake of 123I in a normal population.

Iodine 131 for thyroidal uptake and scan exposes the thyroid to potentially carcinogenic doses of radiation. Iodine 123, on the other hand, compares equally with 131I for uptake studies yet reduces thyroid radiation exposure substantially. Reported fluctuations in normal thyroidal iodine uptake over the past decade, as well as possible radiation injury with 131I, prompted examination of uptake values in a normal population using 123I. The normal range defined in 100 euthyroid subjects was 1% to 8.8% at two hours, and 4% to 27% for the 24-hour uptake. These results are significantly lower than observed eight years ago in this area. No relationship was noted between uptake values and thyroid indices, age, sex, ponderal index, estrogen ingestion, or urinary iodine excretion. Purified 123I appeared to provide clinically reliable results with a substantial reduction in potential radiation injury.