Thyroid Scintigraphy (Iodine-123)

8.2.1 Radiopharmaceutical:

• Na[¹²³I]I
• [¹²³I]Sodium Iodide

8.2.2 Uptake mechanism/biology of the tracer

Administrated iodide is actively taken by a dedicated sodium/iodide symporter (NIS) located on the membrane of the thyroid follicular cell. It is rapidly oxidized and transferred to some tyrosyl residues of the Thyroglobulin molecule. This process is called organification. The iodide is eliminated by urinary excretion.

8.2.3 Indications

The most common indication in adults and adolescents include

• to differentiate causes of primary hyperthyroidism
• to assess the nature/functionality of a nodule, that co-exist with low thyroid stimulating hormone (TSH).
• to differentiate congenital hypothyroidism Na[¹²³I]I allows the perchlorate discharge test to be performed in order to detect abnormal iodide organification with an eutopic thyroid.
  
  Thyroid uptake indicates the rate of thyroid hormone synthesis, and, indirectly, the rate of hormone secretion. When a therapy is planned, the uptake is used to calculate the Na[¹²³I]I therapeutic activity.

8.2.4 Contra-indications

• Pregnancy.
• Cessation of breast feeding is recommended following the administration of Na[¹²³I] [3].

8.2.5 Clinical performances

Thyroid scintigraphy has been used for decades to explore thyroid dysfunction. It is the only imaging modality which correlates anatomy and function.

8.2.6 Activities to administer

The suggested activity to administer is

• Na[¹²³I]I: 8 MBq

In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/). The minimum recommended activity to administer is 3 MBq.

8.2.7 Dosimetry

The effective dose for Na[¹²³I]I is 150 µSv/MBq (low uptake, iv administration) [3]. The organ with the highest absorbed dose is the thyroid: 2.7 mGy/MBq.

The effective dose for Na[¹²³I]I is 1.2 mSv per procedure. In neonates, the ED is about 22 mSv when the injected activity is 3 MBq. (EANM paediatric dosage card)

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."

8.2.8 Interpretation criteria

A normal finding shows bilobed thyroid gland thyroid located at the base of the neck. with homogenous and symmetrical uptake in the lobes, joined inferiorly and medially by the isthmus. Rarely pyramidal lobe is presented.

In a nodular thyroid gland, we evaluate the position of a nodule (upper, middle, lower part of the lobe) and its functionality:

• hot nodule (without any uptake in the normal thyroid tissue)
• warm nodule (with greater uptake than the normal thyroid tissue)
• cold nodule (with lower uptake than the normal thyroid tissue)
• isofixated (same uptake as the normal thyroid tissue
• The normal uptake (normal TSH) ranges between 10 and 15% 2 h after iodine administration.
• Congenital hypothyroidism can be characterised by:
• athyreosis (absent thyroid tissue)
• ectopic (usually lingual at the base of the tongue), or
• normally located but hypoplastic. When a normally shaped thyroid is in the thyroid lodge, a perchlorate discharge test is performed. In case of organification defect, a decrease of Na[¹²³I]I uptake is observed 1 h after perchlorate administration.

8.2.9 Patient preparation

The uptake of Na[¹²³I]I could be decreased by recent administration of iodinated contrast materials, intake of stable iodine in any form, or anti-thyroid agents and other drugs. So, the patient should be questioned carefully regarding diet, previous medications, and radiographic exams prior to administering the radiopharmaceutical. If necessary, the examination should be delayed.

8.2.10 Methods

Additional information can be found in the EANM/SNMMI guideline [147].  

Thyroid scintigraphy is usually performed between 2 and 4 h after injection of the radiopharmaceutical. The patient is scanned supine with the neck in hyperextension. Most nuclear medicine departments use a γ camera equipped with a pin-hole collimator for imaging with an image duration ranging from 5 to 15 min. The location of palpable nodules is confirmed with a radioactive point source. The uptake is usually measured between 2 and 4 h post Na[¹²³I]I injection with a LEHS collimator.

In neonates, anterior images are acquired 1 h post tracer injection, during 5 min, with a LEHS collimator. Given the small size of newborns, the whole body is displayed on a single field. Complementary acquisitions (pinhole or profile) depend on the results of the anterior image.