What is the purpose and protocol of a thyroid scan in diagnosing thyroid dysfunction?

Thyroid Scan: Purpose and Protocol in Diagnosing Thyroid Dysfunction

A thyroid scan is primarily indicated for evaluating thyrotoxicosis to determine its underlying cause, and has limited utility in evaluating thyroid nodules or hypothyroidism. 1

Indications for Thyroid Scan

Primary Indications

• Thyrotoxicosis evaluation
  • Differentiating between Graves' disease, toxic adenoma, toxic multinodular goiter, and thyroiditis 1
  • Particularly valuable when laboratory diagnosis is ambiguous or when toxic adenoma/multinodular goiter is suspected 1
  • Planning radioactive iodine (RAI) therapy 1

Secondary Indications

• Goiter with thyrotoxicosis
  • Confirms that the entire goiter consists of thyroid tissue 1
  • Helps identify hypofunctioning or isofunctioning nodules that may require biopsy 1

Not Indicated For

• Euthyroid nodule evaluation
  • Limited utility as most nodules appear "cold" and most cold nodules are benign 1
  • Low positive predictive value for malignancy 1
• Hypothyroidism workup
  • No role in differentiating causes of hypothyroidism 1
  • All causes of hypothyroidism will show decreased radioiodine uptake 1

Protocol for Thyroid Scan

Radiopharmaceuticals

• Iodine-123 (I-123) - preferred agent due to superior imaging quality 1
• Technetium-99m (99mTc) - alternative when evaluating nodules with suppressed TSH 2

Patient Preparation

• Avoid iodine-containing medications and contrast agents for 4-8 weeks prior to scan 1
• Discontinue thyroid medications as appropriate:
  • T4 (levothyroxine): 4-6 weeks before scan
  • T3 (liothyronine): 2 weeks before scan
  • Antithyroid drugs: 3-5 days before scan

Procedure

1. Radiotracer administration
   • Oral administration for I-123
   • Intravenous injection for 99mTc
2. Uptake measurements
   • For I-123: measurements at 4-6 hours and 24 hours
   • For 99mTc: measurements at 20-30 minutes post-injection
3. Imaging acquisition
   • Anterior, right anterior oblique, and left anterior oblique views
   • Additional views as needed

Interpretation of Results

Patterns and Clinical Correlations

• Diffusely increased uptake: Graves' disease, Hashimoto's thyroiditis in hyperthyroid phase 3
• Focal increased uptake ("hot" nodule): Autonomous functioning thyroid nodule 4
• Focal decreased uptake ("cold" nodule): Various etiologies including benign and malignant lesions 1
• Patchy uptake: Multinodular goiter 1
• Decreased global uptake: Thyroiditis, exogenous thyroid hormone, iodine excess 1

Diagnostic Value in Specific Conditions

• Thyrotoxicosis: 90% sensitivity, 100% specificity for differentiating causes 1
• Nodular disease: Low positive predictive value for malignancy in cold nodules 1

Integration with Other Diagnostic Methods

Complementary Tests

• Ultrasound: First-line imaging for thyroid morphology 1
  • Superior for nodule characterization and risk stratification
  • Should be used in conjunction with scan for multinodular goiter 1
• Laboratory tests: TSH, free T4, T3, thyroid antibodies
  • Low TSH (<0.1 mIU/L) should prompt consideration for scan 5
  • Normal/elevated TSH generally eliminates need for scan 5

Clinical Pitfalls and Considerations

• False results: Recent iodine exposure (contrast media, medications) can affect uptake 1
• Hashimoto's thyroiditis: Can mimic various thyroid disorders on scan, including Graves' disease 3
• Pregnancy/breastfeeding: Contraindicated due to radiation exposure
• Cost-effectiveness: Scan should be reserved for specific indications rather than routine screening 1
• Interpretation challenges: Scan findings should always be correlated with clinical and biochemical data 3

Algorithmic Approach to Thyroid Scan Use

1. Measure TSH first in all patients with suspected thyroid dysfunction
2. If TSH is suppressed (<0.1 mIU/L): Consider thyroid scan to determine cause of thyrotoxicosis 5
3. If TSH is normal or elevated: Thyroid scan generally not indicated 1
4. For nodular thyroid disease:
   • Start with ultrasound for all nodules 1
   • Consider scan only if TSH is suppressed 2, 5
   • Recognize that 97% of patients with normal/elevated TSH will not have hyperfunctioning nodules 5

By following this evidence-based approach, thyroid scans can be appropriately utilized to provide valuable diagnostic information while avoiding unnecessary radiation exposure and healthcare costs.