Procedures
RAIU preparation begins with limiting the patients’ intake of iodine and avoiding interfering medications to maximize I-123 uptake by thyroid tissue.[4] For RAIU alone, 3.7 MBq (100 µCi) of Sodium Iodide I-123 is administered orally. An uptake probe (2-inch diameter), with non-radioactive thallium, activated sodium iodide (NaI (Tl)) crystal (approximately 2 inches thick), a flat field collimator, photomultiplier tube detector, and multi-channel analyzer are used for counting of the 159 keV gamma peak. If rapid iodine turnover is suspected in hyperthyroidism, RAIU may be done at 4-6 hours in addition to the routine 24 hours delay. The radioactive dose is initially placed in a lucite neck phantom with the same geometry as the patient’s neck and counted prior to administration. A second nearly identical Standard dose, which is not administered to the patient, may also be used in the phantom when subsequent counts of the neck are taken. The patient is positioned either sitting or supine with neck extended, and the probe positioned at a constant distance of 20 cm to 30 cm. Counts are also obtained of room background and of the patient’s mid-thigh away from the urinary bladder for the patient background.
If no second standard dose is used, decay corrected 24 hours (24h) RAIU is calculated:
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RAIU (in %) = {(24h Neck – 24h Thigh) / ([Initial Dose in Phantom counts – Initial Room Background] x I-123 decay correction factor for 24h)} x 100%
If a second nearly identical standard dose is used, the 24h calculation RAIU is calculated factoring in the standard dose:
RAIU (in %) = {(24h Neck – 24h Thigh) / (|Correction factor to adjust for difference in activity between initial patient dose and initial Standard dose| x [24h Standard in Phantom – 24h Room Background])} x 100%
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In the absence of an uptake probe, a nuclear scintillation gamma camera with a low energy parallel hole collimator can be used if validated by comparison with a reliable standard.[5][6] For the perchlorate discharge test in neonatal hypothyroidism, RAIU is performed before and after perchlorate administration.[7]
For combined thyroid RAIU uptake and scan, the oral dose is 7.4 to 14.8 MBq (200 to 400 µCi). RAIU is done as above, while for the scan, the thyroid is imaged using either a conventional or small field gamma camera with a pin-hole or parallel-hole collimator centered at 159 KeV after 24 hrs. Imaging is captured with the patient lying supine with the neck extended.
Thyroid radioactive uptake can also be measured using intravenous technetium-99m (Tc-99m) pertechnetate. However, Tc-99m pertechnetate is trapped, but not organified, and may not always reflect true overall thyroid function. RAIU can also be measured using I-131 sodium iodide, which, like I-123 sodium iodide, is both trapped and organified. I-131 has a half-life of 8 days and has both a beta minus emission and a principal 364 keV gamma photon emission; thus, patient ionizing radiation exposure is much higher with I-131. RAIU with I-123 sodium iodide is much more popular, if available.[6][5]
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