RAUI measures thyroidal uptake of iodine after an oral dose of which isotope?

• A. Iodine-131
• B. Iodine-125
• C. Iodine-123
• D. Iodine-127

Answer: (A)

Thyroidal uptake testing relies on the thyroid’s natural ability to trap iodine. To quantify how much iodine the thyroid takes up, a small oral dose of a radioactive iodine isotope is given and the radioactivity in the neck is measured at a fixed time point. The isotope chosen must provide a detectable gamma signal for external counting; iodine-131 has historically been used for this purpose because its gamma emissions are easily measured and the dose can be controlled for diagnostic uptake studies. Although iodine-123 is often preferred today due to lower radiation exposure and favorable imaging properties, the classic and commonly tested choice for this measurement is iodine-131. The other listed isotopes are not typically used for this uptake test: iodine-125 and iodine-127 either do not provide the ideal gamma signal for external measurement or are not suitable (iodine-127 is stable).