Iodine-123 (I¹²³ or I-123) is a radioisotope of the element iodine (atomic number 53) used in nuclear medicine imaging including to scan the thyroid gland. 

Production

Iodine-123 is produced in a cyclotron by bombarding Xenon-124 with a proton. Xenon-124 either will absorb the proton and loses a proton and a neutron to form Xenon-123, or Xenon-124 will absorb the proton and loses two neutrons to form Caesium-123. Caesium will later decay into Xenon-123 while Xenon-123 will decay in Iodine-123 ¹.

Uses, dosages, and time of Imaging

• standard scan: 3.7-14.8 MBq (100-400 μCi) PO, image at 4-6 or 24 hours

• thyroid cancer scan: 55.5 MBq (1.5 mCi) PO, image at 4-6 or 24 hours

Physical properties

• photon energy: 159 keV, gamma rays ¹

• physical half life: 13.2 hours ¹

• decay: internal conversion of electron (gamma energy absorbed by orbital electron and the excited electron gets ejected from the atom) ¹

Normal distribution

• thyroid gland (target organ)

• nasopharynx

• salivary glands

• stomach (target organ)

• colon

• bladder (target organ)

• lactating breasts

Pharmacokinetics

Iodine-123 is rapidly absorbed from the gastrointestinal tract, with detectable activity in the thyroid gland within minutes. It reaches the thyroid follicular lumen in 20-30 min, with rapid trapping and organification.

Excretion

Excretion is primarily renal. It should be noted that it is also secreted in breast milk and crosses the placenta. As such it is contraindicated in pregnancy and requires precautions if breast feeding.