Thyroid

I-123 vs. Tc- pertechnetate

• I-123 shows both trapping and organification while Tc-pertechnetate shows only trapping
• I-123 more expensive but provides concurrent radioiodine uptake and images with a relatively low radiation dose
• Tc-pertechnetate requires no prior preparation of the patient
• Tc has lower radiation thus higher dose can be used leading to faster scan time
• Tc shows uptake in the salivary glands (unless there's increased thyroid uptake such as in Graves disease)

Indications for uptake vs scan

• Scan requires higher dose compared to uptake alone
  

Medications that may decrease iodine uptake

No contrast for 1-2 months prior

Measurements

• Each lobe should measure about 2 x 5 cm
• Slight asymmetry common, right usually bigger than left
• Isthmus can show relatively decreased activity