Medical Applications
123I is the most suitable isotope of iodine for the diagnostic study of thyroid diseases. The half-life of approximately 13.3 h (hours) is ideal for the 24-h (hour) iodine uptake test and 123I has other advantages for diagnostic imaging thyroid tissue and thyroid cancer metastasis. The energy of the photon, 159 keV, is ideal for the NaI (sodium iodide) crystal detector of current gamma cameras and also for the pinhole collimators. It has much greater photon flux than I-131. It gives approximately 20 times the counting rate of I-131 for the same administered dose. The radiation burden to the thyroid is far less (1%) than that of 131I. Moreover, scanning a thyroid remnant or metastasis with 123I does not cause "stunning" of the tissue (with loss of uptake), because of the low radiation burden of this isotope. (For the same reasons, I-123 is never used for thyroid cancer or Graves disease treatment, and this role is reserved for I-131.)
Iodine-123 is supplied as sodium iodide (NaI), sometimes in basic solution in which it has been dissolved as the free element. This is administered to a patient in capsule form, by intravenous injection, or (less commonly due to problems involved in a spill) in a drink. (Iodine-131 is usually administered in a drink, due to the heavy radiation dose to local tissues which results before a capsule could dissolve). The iodine is taken up by the thyroid gland and a gamma camera is used to functional images of the thyroid for diagnosis. Quantitative measurements of the thyroid can be performed to calculate the iodine uptake (absorption) for the diagnosis of hyperthyroidism and hypothyroidism. Dosing can vary; a small dose can start at 11.1 MBq, while it is commonly an amount such as 2-4 mCi. There is a study that indicates a given dose can effectively result in effects of an otherwise higher dose, due to impurities in the preparation. The dose of radioiodine I-123 is typically tolerated by individuals who may be otherwise allergic to iodine, such as those who cannot tolerate contrast mediums containing larger doses of iodine such as used in CT scan, intravenous pyelogram (IVP) and similar imaging diagnostic procedures.
Iodine-123 is also used as a label in other imaging radiopharmaceuticals e.g.metaiodobenzylguanidine (MIBG).
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