Thyroid Scintigraphy (Iodine-131)

8.3.1 Radiopharmaceutical:

• Na[¹³¹I]I (Sodium[¹³¹I]iodide)

8.3.2 Uptake mechanism/ biology of the tracer

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).

8.3.3 Indications

• Same indications as for a thyroid scan for benign diseases with Na[^99mTc]TcO₄ or Na[¹²³I]I only when these radiopharmaceuticals (or Iodine-123) are not available.
• To perform diagnostic whole-body scan for evaluation the stage of the disease, before administering ablative dose.
• To perform therapeutic whole-body scan. Follow up of differentiated thyroid carcinoma: radioiodine scanning to identify thyroid remnant and metastases after Na[¹³¹I]I ablation. This indication is limited nowadays, because rh-TSH stimulated thyroglobulin and ultrasonography are more informative exams in the follow up thyroid cancer.

8.3.4 Contra-indications

Absolute contra-indications:

• Pregnancy and breastfeeding.
• Diagnostic purposes in children under 10 years except in thyroid cancer.

8.3.5 Clinical performance

Follow up of papillary and follicular thyroid carcinoma including metastatic disease after total thyroidectomy [148].

8.3.6 Activities to administer

The suggested activity to administer is

• Na[¹³¹I]I: 75-185 MBq

No recommendations are given for paediatric nuclear medicine.

8.3.7 Dosimetry

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.".

8.3.8 Interpretation criteria/major pitfalls

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.

8.3.9 Patient preparation

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.

8.3.10 Methods

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.