Question

4. a) Describe the measurement of thyroid uptake following the administration of radioiodine (iodine-123) to a patient. 14] b) Calculate the thyroid uptake for a patient if the administered iodine-123 activity was 40 MBq, the aliquot standard activity was 2 MBq and the average patient, standard and background measurements are 63945, 12998 and 200 counts respectively (assume same counting time for all measurements). c) Calculate the equivalent dose to the thyroid if the absorbed dose was 5 mGy assuming pure gamma emissions of iodine-123. How would this differ if the absorbed dose was delivered by alpha particles? 13]

Answer 1

Thyroid uptake determination is a method to measure the fraction of radioactive iodine accumulated in the thyroid after certain intervals of its ingestion. After 18-24 hrs of administration of radioactive iodine, the measurement of thyroid uptake can be performed. It can also be done between 2 to 6 hrs of radioactive iodine ingestion. The measurement is done between the face of the crystal (25- 30cm) and phantom or anterior neck. At each counting procedure, various counts like neck count, count of neck phantom lower thigh counts and background room counts are taken. Radioactive iodine in the neck phantom can be counted before the oral administration and then counts can be corrected in subsequent counting sessions of the patient.

Radioactive iodine uptake (RAIU)

$RAIU = \frac{neck counts (cpm)- thigh counts (cpm)}{admin counts (cpm)- background counts(cpm)}\times 100$

Thyroid uptake determination is a method of differentiating painless and subacute hyperthyroidism resulting from autoimmune “graves disease” and hyperthyroidism from other forms to confirm the hyperthyroidism occurring due to graves disease

Iodine -123 decays by capturing an electron and has a half-life of 13.2 hrs. photons with the energy of 159 KeV are used for imaging and detecting the I -123.

Normal individuals accumulate approx 8 to 35 % of iodine administered in the thyroid glands.

< 8% - hypothyroid;

8-35% euthyroid;

>35% hyperthyroid