Last revised by Andrew Murphy on 3 Apr 2020

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

  • 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
  • photon energy: 159 keV
  • physical half life: 13.2 hours
  • thyroid gland (target organ)
  • nasopharynx
  • salivary glands
  • stomach (target organ)
  • colon
  • bladder (target organ)
  • lactating breasts

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 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. 

ADVERTISEMENT: Supporters see fewer/no ads