CS 
[123I]sodium iodide, [123I]NaI
Author of the review: Ing. Ondřej Lebeda, Ph.D.
[123I]sodium iodide is a very good diagnostic agent in functional and morphological studies of the thyroid gland, using scintigraphy of the uptake test. Compared with [99mTc]sodium pertechnetate, which can be used for the same purpose, the quality and sensitivity of thyroid imaging with [123I]sodium iodide are significantly higher. It is an ideal diagnostic agent when planning 131I therapy of thyroid carcinoma. To date, a smaller amount of [131I]sodium iodide is often used for these purposes; however, it causes a radiation burden on the thyroid more than 2 orders of magnitude higher, and thus demonstrably reduces the uptake of the therapeutic dose of 131I.
Properties
The diagnostic radiopharmaceutical [123I]sodium iodide is essentially an aqueous solution of a non-weighable quantity of sodium iodide which, instead of the stable isotope of iodine, contains its radioisotope 123I. This radioisotope has a half-life of 13.27 h. Unlike the commonly used 131I (half-life 8.01 d), during its transformation it emits no particulate radiation, only gamma radiation of an energy ideal for imaging on SPECT cameras (159 keV). The radiation burden on the body is therefore more than 2 orders of magnitude lower per unit of administered activity compared with 131I (only 23 mSv/MBq at 55 % accumulation in the thyroid gland). Moreover, thanks to the optimal energy of the emitted gamma radiation, activities approximately 4× lower than in the case of 131I are sufficient for good-quality thyroid imaging. The overall radiation burden on the body is thus roughly 500× lower compared with 131I.
Because iodine is deposited in the thyroid gland very rapidly and selectively, even in the case of oral intake, [123I]sodium iodide is an exceptionally suitable radiopharmaceutical for thyroid imaging, both in terms of the quality of the imaging itself and because of the very low radiation burden on the patient.
Pharmacokinetics
After oral administration, absorption of iodide is essentially complete within 1–2 hours, but it may be slowed by food present in the stomach.
Distribution after the drug enters the systemic circulation leads to dominant accumulation of iodide in the thyroid gland, with a maximum 24–48 hours after administration. As early as 5 hours after ingestion, 50 % of the maximum concentration in the thyroid gland is reached. These ratios may be affected by factors such as age, thyroid status, or the degree of renal iodide clearance. Accumulation may be further affected by concomitant medication.
The biological half-life of iodide removal from the thyroid gland has been estimated at 80 days (adult), and at 65, 50, 40 and 30 days for children aged 15, 10, 5 and 1 year respectively. The time window for diagnostic administration of 123I iodide is therefore governed by the physical half-life (13.27 h).
From all other organs, iodide is removed with a half-life of renal clearance, which is about 8 hours. Iodide is eliminated from the body 90 % in the urine, the remainder in the faeces and sweat. Within 24 hours, about 50 % of the iodide not bound in the thyroid gland is excreted in the urine.
A small amount of iodide is taken up by the salivary glands, the gastric mucosa, breast milk, the placenta and the choroid plexus. The thyroid gland removes almost 20 % of the iodide from the blood on the first pass. The accumulated iodine is incorporated in the thyroid gland into organic compounds from which thyroid hormones are formed. Iodine is also released intrathyroidally from iodinated amino acids not used in hormone synthesis. The iodine level in the thyroid gland is relatively high (10 mg) and has a slow turnover.
Toxicity
The acute toxicity of iodide was tested after oral administration of sodium iodide to rats and mice. The LD50 values found were 4340 mg/kg and 1000 mg/kg respectively. After intravenous administration, the LD50 value is 1060 mg/kg for rats and 760 mg/kg for dogs.
The safety factor is greater than 106, given the low administered quantity of the compound (< 0.1 µg for diagnosis). For the diagnostic administration of 123I, no relevant toxic effects of very high doses of sodium iodide are known. No data are available from animal studies concerning toxicity after repeated administration or reproductive toxicity. No teratogenic or mutagenic effects are known.
Contraindications are also not known. A relative contraindication may be a disease accompanied by impaired absorption from the gastrointestinal tract, in which the absorbed dose from ionising radiation increases.
Use
[123I]sodium iodide is a very good diagnostic agent in functional and morphological studies of the thyroid gland, using scintigraphy of the uptake test.
The recommended administered activity for adults (70 kg) ranges between 3.7 and 18 MBq. A lower activity (3.7 MBq) is recommended for use in the uptake test, a higher one (11.1–14.8 MBq) for scintigraphy. For the examination of patients after surgical removal of the thyroid gland, 14–18 MBq is administered. 123I must be administered orally in solution. Imaging is performed 4–6 hours (16–24 hours) after administration. To calculate the degree of accumulation of 123I in the thyroid gland, one of the reliable standard procedures must be used (usually those recommended by the manufacturer of the given gamma camera).
In small children, when calculating the administered activity for scintigraphy, the value A(adult) = 14.8 MBq must be used if the imaging is to be of sufficient quality.
Scientific literature
Neumann DR, Obuchowski NA, Difilippo FP: Preoperative 123I/99mTc-sestamibi subtraction SPECT and SPECT/CT in primary hyperparathyroidism. J Nucl Med, 2008. PubMed
Fugazzola L, Persani L, Vannucchi G, Carletto M, Mannavola D, Vigone MC, Cortinovis F, Beccaria L, Longari V, Weber G, Beck-Peccoz P: Thyroid scintigraphy and perchlorate test after recombinant human TSH: a new tool for the differential diagnosis of congenital hypothyroidism during infancy. Eur J Nucl Med Mol Imaging34(9):1498-503, 2007. PubMed
Schoen EJ, Clapp W, To TT, Fireman BH: The key role of newborn thyroid scintigraphy with isotopic iodide (123I) in defining and managing congenital hypothyroidism. Pediatrics 114(6):e683-8, 2004. PubMed
Hilditch TE, Dempsey MF, Bolster AA, McMenemin RM, Reed NS: Self-stunning in thyroid ablation: evidence from comparative studies of diagnostic 131I and 123I. Eur J Nucl Med Mol Imaging 29(6):783-8, 2002. PubMed
LaFranchi S: Congenital hypothyroidism: etiologies, diagnosis, and management. Thyroid 9(7):735-40, 1999. Review. PubMed
Morita S, Umezaki N, Ishibashi M, Kawamura S, Inada C, Hayabuchi N: Determining the breast-feeding interruption schedule after administration of 123I-iodide. Ann Nucl Med 12(5):303-6, 1998. PubMed
