The pharmacological active substance is Iodine-131 as sodium iodide that is taken up by the thyroid (same physiology as for Iodine-123, but its radiation burden is much higher due to high energy beta radiation).
Absolute contra-indications:
Follow up of papillary and follicular thyroid carcinoma including metastatic disease after total thyroidectomy [148].
The suggested activity to administer is
No recommendations are given for paediatric nuclear medicine.
The effective dose for Na[131I]I ranges between 0.28 mSv/MBq (thyroid blocked, oral administration) and 14 mSv/MBq (low thyroid uptake, oral administration) [3]. The organ with the highest absorbed dose is the thyroid: 2.2-280 mGy/MBq.
The effective dose for Na[131I]I ranges between: 21–2590 mSv per procedure.
Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population.".
Na[131I]I whole body scintigraphy demonstrates some activity in the remnants and nonspecific uptake in the salivary glands, the stomach, intestine, and urinary bladder.
Na[131I]I whole body scintigraphy includes planar AP and PA images of the whole body, spot images of the neck, mediastinum, and on any abnormal focus of Na[131I]I uptake,
-SPECT/CT (dual head single photon emission computed tomography) image of the neck region to differentiate uptake in normal thyroid remnants from lymph node metastases (especially when thyroid remnants are large),
-SPECT/CT of the thorax to differentiate uptake in lung metastases from rib lesions
-SPECT/CT of abdomen to differentiate accumulation of radioiodine in intestine or bladder from a pelvic bone lesion.
Thyroid hormone (Levothyroxine) should be withdrawn for 3-4 weeks prior to diagnostic testing and Na[131I]I administration. RhTSH (Thyrogen™) can be used instead of thyroxine withdrawal.
Many pharmacological agents are known to interact with Na[131I]I . Therefore, it is necessary to obtain a full drug history and ascertain whether any medications are required to be withheld prior to the administration of Na[131I]I , e.g. iodine contrast agents, amiodarone, etc.
Additional information can be found in the EANM/SNMMI guideline [147].
Whole body scintigraphy may be performed after the administration of a diagnostic dose, usually 75-185 MBq of Na[131I]I (2-5 mCi). This includes a whole-body scan and SPECT/CT images of any abnormal uptake focus.