When should a nuclear medicine (NM) thyroid uptake scan be ordered for patients with thyroid nodules?

When to Order NM Thyroid Uptake on Thyroid Nodules

Nuclear medicine thyroid uptake scans should be ordered only when a patient with a thyroid nodule has a subnormal TSH level to determine if the nodule is hyperfunctioning ("hot") or when the etiology of thyrotoxicosis needs to be determined. 1, 2

Appropriate Indications for NM Thyroid Uptake Scans

Primary Indication: Subnormal TSH

• When evaluating thyroid nodules, the first step is always ultrasound followed by TSH measurement 2
• If TSH is subnormal (low), a radionuclide thyroid uptake scan is indicated to determine nodule functionality 1, 3
• Hyperfunctioning ("hot") nodules identified on scan are rarely malignant and do not require fine needle aspiration (FNA) 3

Secondary Indications

• To differentiate between causes of thyrotoxicosis when clinical presentation is ambiguous 1:
  • Graves' disease (diffuse uptake)
  • Toxic adenoma (focal uptake in single nodule)
  • Toxic multinodular goiter (multiple areas of increased uptake)
  • Destructive thyroiditis (very low uptake)
• To aid in planning radioactive iodine (RAI) therapy for hyperthyroidism 4
• To identify hypofunctioning or isofunctioning nodules in multinodular goiters that may require biopsy 1

When NOT to Order NM Thyroid Uptake Scans

• For euthyroid patients (normal TSH) with thyroid nodules 1, 2
• For initial evaluation of suspected goiter without associated thyrotoxicosis 1
• For evaluation of hypothyroidism (all causes show decreased uptake) 1
• As a screening tool for thyroid cancer 2
• For nodules already characterized as suspicious by ultrasound criteria in euthyroid patients 2

Practical Algorithm for Thyroid Nodule Evaluation

1. Perform dedicated thyroid ultrasound for all thyroid nodules
2. Measure serum TSH
3. If TSH is subnormal:
   • Order radionuclide thyroid uptake scan
   • If nodule is "hot" (hyperfunctioning): No FNA needed, consider treatment options for hyperthyroidism
   • If nodule is "cold" (non-functioning): Proceed with risk stratification based on ultrasound features
4. If TSH is normal or elevated:
   • Skip radionuclide scanning
   • Use ultrasound risk stratification systems (e.g., ACR TI-RADS) to determine need for FNA

Important Clinical Considerations

• I-123 is preferred over I-131 for thyroid scanning due to superior imaging quality 1
• Although "cold" nodules have higher malignancy risk than "hot" nodules, most cold nodules are still benign, resulting in low positive predictive value of scan alone 1
• Doppler ultrasound may be an alternative to nuclear medicine for differentiating causes of thyrotoxicosis but is less preferred as it measures blood flow rather than direct thyroid activity 1
• In multinodular goiters with thyrotoxicosis, scan results should be compared with ultrasound to identify nodules requiring biopsy 1

By following this evidence-based approach, unnecessary radiation exposure can be avoided while ensuring appropriate evaluation of thyroid nodules for malignancy risk and functional status.