REF007 - Thyroid Imaging and Uptake Measurement with I-123

The purpose of this document is to assist clinicians in deciding on the appropriateness of a nuclear medicine thyroid scan using iodine-123 and to give guidance on the mechanism of referral

The main functions of the thyroid gland are to concentrate and organify inorganic iodine, to store these iodinated compounds and then to release them as active hormones into the circulation. Functioning thyroid tissue is best identified using nuclear medicine techniques.

In most nuclear medicine studies the radiopharmaceutical Tc^99m pertechnetate is used. Thyroid tissue concentrates Tc^99m but does not organify it into thyroid hormones. A thyroid scan with Tc^99m essentially provides a display of the functioning trapping mechanism of the thyroid gland. A thyroid scan with I¹²³ is a more prolonged study but further shows the organification of I¹²³ into thyroid hormones by the functioning thyroid tissue. This type of scan is therefore useful in particular cases. Some thyroid cancers merely trap but do not organify iodine. In this case there would be increased uptake on a Tc^99m pertechnetate scan but reduced uptake on an I¹²³ scan.

I¹²³is not readily available, it is expensive and it gives a higher radiation dose to the patient than Tc^99m pertechnetate. The investigation is more prolonged as the I¹²³ is organified slowly by the thyroid gland. A typical uptake of 30% I¹²³ at 24 hours following administration is expected.

I¹²³ thyroid imaging is therefore restricted to specific cases where the diagnostic question relates to the presence or absence of organifying thyroid tissue.

• Any clinician holding a current valid medical qualification currently employed in secondary care.
• Referrals from primary care will be accepted if the investigation has been suggested by a Consultant Radiologist in a report from a previous investigation or following discussion with a secondary care specialist. Details of such suggestions or discussions should be included on the request form.

Referring clinicians are reminded of the requirement to avoid unnecessary exposure to ionising radiation. The investigation may already have been requested at this hospital or elsewhere and clinicians are asked to be particularly vigilant to ensure that their request is not a double request.

Requests will only be accepted if the diagnostic question fulfills one of the referral criteria detailed in the following paragraph.

Appropriate clinical information must be supplied to allow the investigation to be justified. It is not acceptable to simply request the investigation without supplying appropriate clinical information.

Requests will be returned to the referrer if:

• The diagnostic question is unclear.
• The diagnostic question cannot be answered by the investigation.
• Insufficient clinical information is supplied to justify the investigation.

Requests should be submitted on the orange stripe NUCLEAR MEDICINE (ISOTOPE SCAN) request form. Patient details, investigation required and reason for test/diagnostic question should be fully and appropriately completed. The form MUST be signed.

• The referrer should state clearly on the request form that they are happy for thyroxine to be withdrawn for the required period.
• Note the requirement for Patient Preparation below

Sufficient clinical information must be provided to justify the investigation proceeding.

I¹²³ is restricted to the specific clinical indications detailed below.

• Follow a low iodine diet for 2 weeks prior to imaging.^[1]

• Patients taking Amiodarone may need to withdraw this for 3-6 months prior to imaging.^[1] If the patient is taking Amiodarone, this must be discussed with the referring clinician and/or cardiology and the ARSAC licence holder prior to the patient being appointed for imaging.
• Avoid iodinated intravenous contrast media (as used in CT and X-ray procedures) for at least 8 weeks prior to imaging.^[1]
• Avoid medications containing iodine (e.g. iodinated contrast agents, antiseptics, eye drops, iodinated multivitamins or mineral supplements, kelp, Lugol's solution, potassium iodide and expectorants) for at least 4 weeks prior to imaging.^[1]

• Stop thyroxine (T4) for four weeks prior to imaging. This may be replaced with triiodothyronine (T3) for two weeks.^[1]
• Stop tri-iodothyronine (T3) - whether prescribed to replace the T4 or as a stand alone medication - two weeks before the imaging.^[1]
• Stop anti-thyroid medication (methimazole, carbimazole, propylthiouracil) for a minimum of 7 days before the test^[1][4] . These may be restarted following the scan.

• Ask the patient to remove chains and/or necklaces from their neck prior to imaging.