What is the purpose and procedure of a radioactive iodine uptake scan in diagnosing and treating hyperthyroidism?

Radioactive Iodine Uptake Scan for Hyperthyroidism

A radioactive iodine uptake (RAIU) scan is the preferred diagnostic test for determining the cause of hyperthyroidism, as it directly measures thyroid gland activity and helps distinguish between different etiologies of thyrotoxicosis to guide appropriate treatment decisions. 1

Purpose of RAIU Scan

The primary purposes of radioactive iodine uptake scanning include:

• Differentiating causes of hyperthyroidism (productive vs. destructive thyrotoxicosis)
• Identifying the pattern of thyroid overactivity (diffuse vs. focal)
• Quantifying iodine uptake to calculate therapeutic radioiodine doses
• Detecting ectopic thyroid tissue
• Evaluating thyroid nodule functionality

Clinical Indications

RAIU scanning is particularly valuable in the following scenarios:

• Ambiguous cases of thyrotoxicosis where the cause is unclear
• Suspected toxic adenoma or multinodular goiter
• Planning radioactive iodine (RAI) therapy
• Distinguishing Graves' disease from other causes of hyperthyroidism
• Evaluating thyroid nodules for autonomous function

Procedure Details

Preparation

• Discontinuation of medications that may affect results:
  • Antithyroid drugs (methimazole, propylthiouracil)
  • Thyroid hormones
  • Iodine-containing medications and contrast agents
  • Amiodarone
• Fasting for 2-4 hours before the procedure
• Pregnancy testing for women of childbearing age

Radioisotope Selection

• Iodine-123 (I-123): Preferred for diagnostic imaging due to:

  • Superior imaging quality
  • Lower radiation exposure
  • 13-hour half-life
  • Gamma emission suitable for imaging 1, 2

• Technetium-99m pertechnetate: Alternative option when:

  • I-123 is unavailable
  • Rapid results are needed
  • Note: Only measures trapping (not organification) 2, 3

• Iodine-131 (I-131): Used primarily for therapy rather than diagnosis due to higher radiation dose 2

Testing Protocol

1. Baseline measurement: Background radiation is measured
2. Radioisotope administration: Oral capsule or liquid solution
3. Uptake measurements:
   • 4-6 hour measurement (early phase)
   • 24 hour measurement (standard for most complete assessment)
4. Imaging acquisition: Patient positioned under gamma camera
5. Quantitative analysis: Calculation of percentage uptake
6. Visual interpretation: Assessment of distribution pattern

Interpretation of Results

Normal Uptake Values

• 4-6 hour: 3-16%
• 24 hour: 8-29% (varies by geographic region and iodine intake)

Patterns and Clinical Correlation

1. High uptake patterns (productive thyrotoxicosis):

   • Diffuse increased uptake: Graves' disease
   • Focal increased uptake: Toxic adenoma
   • Multiple focal areas: Toxic multinodular goiter 2

2. Low uptake patterns (destructive thyrotoxicosis):

   • Subacute thyroiditis
   • Postpartum thyroiditis
   • Amiodarone-induced thyroiditis (type 2)
   • Factitious thyrotoxicosis 2, 3

3. Nodule evaluation:

   • "Hot" nodules: Autonomous functioning (rarely malignant)
   • "Cold" nodules: Non-functioning (higher risk of malignancy)
   • "Warm" nodules: Functioning similar to surrounding tissue 1, 2

Clinical Applications in Treatment Planning

• Graves' disease: RAIU helps calculate therapeutic I-131 dose based on gland size and uptake 4
• Toxic nodular disease: Identifies autonomous nodules for targeted therapy 1
• Multinodular goiter: Confirms that entire goiter is thyroid tissue before RAI therapy 1

Advantages Over Other Modalities

While Doppler ultrasound can help differentiate causes of thyrotoxicosis by assessing blood flow (increased in Graves', decreased in thyroiditis), RAIU is preferred because:

• It directly measures thyroid function rather than inferring it from blood flow
• Has superior specificity (100% vs 90%) for diagnosing causes of hyperthyroidism 1
• Provides quantitative data useful for therapeutic planning 2, 3

Limitations and Precautions

• Contraindications:

  • Pregnancy and breastfeeding
  • Recent iodinated contrast administration
  • Recent consumption of iodine-rich foods

• Factors affecting results:

  • Iodine intake/exposure
  • Medications (antithyroid drugs, amiodarone)
  • Renal function (affects clearance)
  • Recent acute illness

Follow-up Considerations

• Patients who undergo subsequent RAI therapy should be monitored for:
  • Development of hypothyroidism (common outcome)
  • Persistent hyperthyroidism requiring retreatment
  • Thyroid nodules that may develop years after treatment 5

RAIU scanning remains an essential diagnostic tool in the evaluation of hyperthyroidism, providing functional information that complements the anatomical data obtained from ultrasound and helps guide appropriate therapeutic interventions.