I-123 Uptake

The radioactive iodine uptake (RAIU) test is used to quantitate the overall metabolism and kinetics of iodine in the thyroid gland by measuring how much orally ingested iodide is concentrated in the thyroid gland. Iodine-123 (I-123) is the most commonly used isotope for RAIU.

Iodine is essential for metabolism as it is required for the intrathyroidal synthesis of triiodothyronine (T3) and thyroxine (T4). Dietary iodide (I-, the ionized form of iodine) is rapidly absorbed by the gastrointestinal tract into plasma. It enters the follicular cells of the thyroid gland through the sodium-iodide symporter (NIS). NIS utilizes a gradient created by sodium-potassium ATPase for cotransport. NIS is predominantly found in the basolateral membrane of the thyroid follicular cells, where it can increase the concentration of the iodide in the thyroid gland up to 40 times the plasma level. NIS is one factor affecting the concentration of iodine accumulated within the thyroid gland, and its expression is regulated by thyroid-stimulating hormone (TSH). Once in the thyroid gland, the iodide is processed through organification into thyroglobulin to produce T3 and T4.

I-123, a radioisotope of iodine, is often used for RAIU together with nuclear medicine thyroid imaging, also known as thyroid scan. I-123 is produced in a cyclotron by bombarding Xenon-124 (Xe-124) or Tellurium-123 (Te-123) with protons. I-123 has a gamma emission of 159 keV and half-life of 13 hours, decaying by electron capture to form Te-123. I-123 Sodium Iodide is taken orally as a pill or liquid, where the iodide is rapidly absorbed in the upper gastrointestinal tract. The radioisotope can be seen concentrating in the thyroid gland within 20-30 minutes. RAIU with I-123 is usually performed 24 hours after administration to reduce background. An additional uptake measurement is sometimes taken at 4 to 6 hrs.