What is the procedure for radioactive thyroid testing?

Radioactive Thyroid Testing: Procedure and Mechanism

Radioactive thyroid testing uses radioactive iodine isotopes (primarily I-123 or I-131) that are administered orally, concentrated by thyroid follicular cells, and then imaged with a gamma camera to assess thyroid function, morphology, and identify functioning versus non-functioning tissue. 1

Core Mechanism of Action

The procedure exploits the thyroid gland's unique ability to concentrate iodine:

• Iodine uptake by follicular cells: The thyroid actively transports iodine from the bloodstream, making it the critical organ for radioiodine concentration 2
• Dual radiation emission: I-131 emits both beta (β-) particles for therapeutic tissue destruction and gamma (γ) rays for diagnostic imaging 2
• Tissue-specific localization: Beta particle penetration is limited to thyroid cells, restricting radiation effects to the target tissue 2

Radiotracer Selection

I-123 is preferred over I-131 for diagnostic imaging due to superior image quality, though both are acceptable 1:

• I-123 advantages: Better imaging characteristics, lower radiation dose to thyroid 1
• I-131 considerations: Higher radiation exposure; absorbed dose triples after two half-lives of decay, potentially exceeding I-131 therapeutic scan doses 3
• Tc-99m pertechnetate: Alternative tracer that provides lower thyroid radiation dose than I-123 (p,2n) at time of delivery 3

Clinical Applications by Indication

Thyrotoxicosis Evaluation

Radionuclide uptake and scan is the preferred initial imaging to distinguish overactive thyroid (Graves disease, toxic adenoma) from destructive thyroiditis 1:

• Confirms diagnosis when laboratory tests (TSH receptor antibodies) are ambiguous 1
• Differentiates causes based on uptake patterns: increased uptake indicates overactive gland, decreased uptake suggests destructive process 1
• Guides radioactive iodine (RAI) therapy planning 1

Multinodular Goiter Assessment

When goiter is associated with thyrotoxicosis 1:

• Confirms entire goiter consists of thyroid tissue
• Identifies hypofunctioning or isofunctioning nodules requiring biopsy when compared with ultrasound 1
• Note: "Cold" nodules on scan have higher malignancy risk, but most cold nodules are benign, resulting in low positive predictive value 1

Post-Thyroidectomy Surveillance for Differentiated Thyroid Cancer

Whole-body scan (WBS) with I-131 or I-123 is performed 2-4 months post-surgery after thyroid hormone withdrawal (4-6 weeks) to induce hypothyroidism and elevate TSH 4:

• TSH elevation stimulates residual thyroid tissue iodide uptake 4
• Post-therapeutic WBS after ablative I-131 doses (100-150 mCi) provides highly sensitive detection 1
• Iodine-containing foods and contrast media must be avoided prior to scanning 4

Alternative preparation: Recombinant human TSH (rhTSH) administration while continuing levothyroxine therapy achieves similar ablation success rates without inducing hypothyroidism 1

Suspected Recurrence Detection

I-123 whole-body scan is usually appropriate for suspected differentiated thyroid cancer recurrence, complementary with ultrasound 1:

• More commonly used in intermediate- and high-risk patients 1
• May be omitted in low-risk patients with undetectable stimulated thyroglobulin and normal neck ultrasound 1

Procedure Protocol

Patient Preparation

• Thyroid hormone withdrawal: Stop levothyroxine 4-6 weeks before scan to achieve TSH >30 mIU/L 4
• Dietary restrictions: Avoid iodine-containing foods and contrast media 4
• Alternative: rhTSH injections (0.9 mg IM for 2 consecutive days) while continuing thyroid hormone 1

Administration and Imaging

• Oral administration: Radioiodine given as capsule or solution 4
• Uptake measurement: Typically performed at 3-4 hours and/or 24 hours post-administration 5
• Imaging timing: Scan performed 24-72 hours after tracer administration 1
• Diagnostic doses: Range from 2-5 mCi for diagnostic scans 6; therapeutic doses 100-150 mCi for ablation 1, 4

Radiation Safety Considerations

High-energy gamma emission from I-131 requires strict radiation protection measures 2:

• Proper shielding and distance from radiation source
• Minimizing exposure time for healthcare personnel 2
• Capsule formulations preferred over oral solutions to reduce spill risk and healthcare worker exposure 4
• Treatment may be outpatient (30-50 mCi) or inpatient (>100 mCi) depending on dose 4

Important Limitations

• No role in hypothyroidism: All causes show decreased radioiodine uptake; imaging does not differentiate etiologies 1
• No role in medullary thyroid cancer: MTC cells do not concentrate iodine 1
• Detection limits: Even 30 mCi diagnostic doses may miss small metastatic lesions due to background activity and depth 6