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The Radioiodine Uptake Test and Scan

© Elaine Moore

Patients who are initially diagnosed with hyperthyroidism are often ordered to have a radioiodine uptake test (RAI-U) or scan. This is a diagnostic test used to determine how well the thyroid gland takes up or absorbs iodine, and how the iodine is distributed among the various cells in the gland. In the RAI-U, true hyperthyroidism can be distinguished from conditions where patients happen to be taking excess thyroid hormone or have transient thyroiditis due to infections. In true hyperthyroidism, the uptake or amount of iodine absorbed by the thyroid is increased.

For fifty years, the RAI-U test has been used to help diagnose autoimmune hyperthyroidism, which is responsible for more than 90% of all cases of hyperthyroidism in the West. In recent years, however, this test is no longer as widely used. Why is the RAI-U no longer needed? A blood test for stimulating TSH receptor antibodies (also known as thyroid stimulating immunoglobulins or TSI) is more specific for Graves' disease. Furthermore, the blood test doesn't expose the patient and his or her already stressed thyroid to excess radioiodine.

Because the RAI-U is seldom properly explained, many hyperthyroid patients are confused and think they're receiving radioiodine ablation to destroy their thyroids. The RAI-U has a similar principle to radioiodine ablation, but a smaller amount of a different isotope is used for the RAI-U diagnostic test. For ablation, I-131 is usually used, and for the RAI-U, I-125 is used.

For both the test and the ablative procedure, radioiodine is given orally as a dose or drink. In the test, however, the amount of radioiodine taken up by the thyroid gland is measured by an imaging test which is taken at a certain time which can range from 2-24 hours after the patient ingests the radioiodine. At the same time as the RAI-U is measured, a scan may done to show the pattern in which the iodine is distributed throughout the gland. In Graves' disease, the uptake is high, and the pattern of the scan is diffuse. In the case of nodules, there will be spots in the scan with differences in density. In these hot or cold nodules, more iodine or less iodine respectively is taken up than in other areas of the gland.

The normal range for the RAI-U is 8% to 35% although the ranges are different in various geographic locations, relative to dietary iodine concentrations. Some researchers say that with the increased iodine content of the American diet, the RAI-U no longer shows clear abnormalities.