Thyroid scan (I-123)

Thyroid scan (thyroid scintigraphy) is a nuclear medicine examination used to evaluate thyroid tissue. 

  • functional status of a thyroid nodule
  • thyrotoxicosis: differential diagnosis
  • thyroid cancer
    • whole body scan for distant metastases
    • estimation of local residual thyroid post thyroidectomy
    • follow-up for tumour recurrence

Medications that interfere with thyroid uptake of radio-iodine should be discontinued. Review of the history should be carried out to ensure the patient has not received iodine-containing contrast (e.g. for CT or angiography). Patients should be fasted for 4 hours prior to study.

Iodine123 is the commonly used radioisotope.  It is administered orally in capsule form (100-400 microCi). Scanning is performed either at 4-6 or 24 hours.

An alternative radioisotope is Tc-99m pertechnetate. Administration is intravenous, and imaging must be done early (maximum uptake at about 20 minutes).

  • camera: gamma camera
  • collimator: 3-6 mm aperture pinhole collimator
  • window: 20% energy window centered at 159 keV.

The patient is positioned supine with the chin up and the neck extended. The collimator is then positioned so that the thyroid fills about two-thirds of the diameter of the field of view. Mark the chin and suprasternal notch. Note the position and mark palpable nodules and surgical scars. Place marker sources lateral to the thyroid to calibrate size.

Three views are typically obtained: anterior; 45-degree LAO; and 45 degree RAO (move the collimator, if possible, rather than the patient).

Each view should have 100-250k counts.

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Article information

rID: 12707
Synonyms or Alternate Spellings:
  • I-123 thyroid imaging
  • Thyroid scintigraphy

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