radioactive iodine excretion test, a method of evaluating thyroid function that entails measuring the amount of radioactive iodine (RAI) in urine after the patient is given an oral tracer dose of RAI in the form of the isotope 131I. Normally, 5% to 35% of the dose is absorbed by the thyroid; absorption is increased in hyperthyroidism and decreased in hypothyroidism, and the amount excreted in urine is inversely proportional to the uptake of RAI. After administration of the tracer, a scintillation detector is placed over the patient’s neck at 2, 6, and 24 hours to measure the amount of RAI accumulated by the thyroid; the amount excreted is assayed in urine collected for 24 hours after the oral dose. See also radioactive iodine uptake.
