What is the best imaging modality for thyroid evaluation?

Best Imaging for Thyroid Evaluation

Ultrasound is the best and first-line imaging modality for evaluating thyroid disease in nearly all clinical scenarios. 1, 2

Why Ultrasound is Superior

Ultrasound provides the highest sensitivity for detecting thyroid nodules and offers superior specificity for cervical lymph node evaluation compared to all other imaging modalities. 2, 3 The American College of Radiology designates ultrasound as the most appropriate initial imaging for:

• Palpable thyroid nodules in euthyroid patients 2
• Suspected goiter to confirm thyroid origin and characterize morphology 2
• Preoperative evaluation of differentiated thyroid cancer 1
• Characterizing nodule features for malignancy risk stratification 1, 2

Ultrasound detects additional sites of metastatic disease not appreciated on physical examination in 20% of thyroid cancer patients. 1 It has excellent resolution for lymph node morphology and demonstrates sensitivity of 75% with specificity of 92% for cervical lymph node evaluation, far exceeding CT's specificity of only 25%. 3

When to Add CT Imaging

CT should be added as an adjunct to ultrasound, not as a replacement, in specific situations:

• Substernal goiter extension - CT is superior to ultrasound for evaluating deep extension and the degree of tracheal compression 1, 2, 3
• Advanced thyroid cancer with clinical suspicion for invasive primary tumor or bulky lymph node involvement 1, 2
• Laryngeal, tracheal, esophageal, or vascular involvement - CT better delineates the extent of invasion 1, 3
• Central compartment and retropharyngeal lymph nodes - CT has higher sensitivity than ultrasound in these specific locations 1, 3

CT with iodinated contrast is recommended when evaluating for invasive features or bulky nodal disease, as contrast improves assessment of vascular encasement and small nodal metastases. 1 Contrast is not contraindicated for differentiated thyroid cancer based on recent studies showing minimal iodine retention concerns. 1

Role of Nuclear Medicine Imaging

Radionuclide uptake and scan should only be performed when TSH is suppressed (low). 4 This imaging is indicated for:

• Thyrotoxicosis evaluation - to differentiate Graves disease, toxic adenoma, toxic multinodular goiter, and thyroiditis 1, 2, 4
• Planning radioactive iodine therapy 1, 4
• Confirming that goiter tissue is thyroid in origin 1, 4

I-123 is preferred over I-131 due to superior imaging quality. 1 In multinodular goiter, the radionuclide scan should be compared to ultrasound to identify hypofunctioning or isofunctioning nodules requiring biopsy. 1, 4

Critical Pitfalls to Avoid

• Never use radionuclide scanning in euthyroid patients to determine malignancy or decide on biopsy - it wastes resources and has low diagnostic value 4
• Do not rely on CT or MRI alone to differentiate benign from malignant nodules - these modalities have limited utility for this purpose 2
• Always check TSH levels before selecting imaging - the TSH result guides the appropriate imaging pathway 4
• Do not skip ultrasound and proceed directly to CT - ultrasound must be performed first, with CT added only for specific indications 2, 3

MRI and PET/CT Have Limited Roles

MRI is an alternative to CT for goiter evaluation but is less preferred due to respiratory motion artifact. 1, 2 FDG-PET/CT is not recommended for initial thyroid evaluation or preoperative staging of differentiated thyroid cancer due to low sensitivity (30% for cervical lymph node metastases). 1

Practical Algorithm

1. Start with TSH measurement 4
2. Perform ultrasound as first-line imaging regardless of TSH result 2, 4
3. If TSH is low (thyrotoxicosis): add radionuclide uptake scan 2, 4
4. If substernal extension or advanced disease suspected: add CT with contrast 2, 3
5. If TSH is normal or high: ultrasound alone is typically sufficient 4

Ultrasound-guided fine-needle aspiration significantly decreases inadequate samples compared to palpation-guided technique and is particularly beneficial for nonpalpable, multiple, or heterogeneous nodules. 5, 6