What is the appropriate workup for a large thyroid mass?

Workup of a Large Thyroid Mass

Begin with thyroid ultrasound as the initial and essential imaging modality for all patients with a large thyroid mass, followed by fine-needle aspiration (FNA) biopsy for nodules meeting size and sonographic criteria, with CT neck with IV contrast reserved for cases with clinical suspicion of advanced disease or compressive symptoms. 1

Initial Clinical Assessment

Essential Laboratory Tests

• Thyroid-stimulating hormone (TSH) to assess thyroid function and guide further workup 2
• Serum calcitonin if medullary thyroid carcinoma is suspected based on family history or clinical features 1
• Complete blood count and platelets as part of baseline evaluation 1

Physical Examination Focus

• Evaluate for compressive symptoms including dysphagia, dyspnea, or voice changes that suggest tracheal or esophageal involvement 2
• Assess for cervical lymphadenopathy through systematic neck palpation 1
• Screen clinically for myasthenia gravis and other paraneoplastic syndromes in patients where thymoma is in the differential 1

Imaging Algorithm

First-Line: Thyroid Ultrasound

Ultrasound is the primary imaging modality for all thyroid masses and should be performed in every case 1. The ACR guidelines designate this as "usually appropriate" for thyroid nodule evaluation 1.

Key ultrasound objectives include:

• Characterizing nodule size, composition, echogenicity, and margins 1
• Evaluating for extrathyroidal extension and multifocal disease 1
• Assessing cervical lymph nodes, particularly in the lateral compartments, where US has higher diagnostic accuracy than CT 1
• Applying ACR TI-RADS criteria to determine FNA indication 1

When to Add CT Neck with IV Contrast

CT with IV contrast is indicated as an adjunct to ultrasound when there is clinical suspicion for advanced disease 1. This is "usually appropriate" in specific scenarios 1.

Specific indications for CT include:

• Large masses with compressive symptoms (dysphagia, stridor, dyspnea) 2
• Clinical evidence of invasive primary tumor 1
• Clinically apparent multiple or bulky lymph nodes 1
• Need to evaluate substernal or mediastinal extension that cannot be visualized on ultrasound 1
• Assessment of laryngeal, tracheal, esophageal, or vascular involvement 1

CT is superior to ultrasound for delineating the inferior border of disease and determining mediastinal structure involvement 1.

Role of MRI

MRI with and without contrast may be considered when differentiating thyroid malignancy from thymic cyst or thymic hyperplasia in anterior mediastinal masses, as it provides better discrimination than CT and may avoid unnecessary surgery 1. However, for typical thyroid masses, MRI is generally not indicated as a first-line study 1.

PET/CT Considerations

FDG-PET/CT is not routinely recommended for initial thyroid mass workup 1. It may be considered as clinically indicated in cases of aggressive histology or advanced stage disease 1, but should not be part of standard initial evaluation 1.

Tissue Diagnosis

Fine-Needle Aspiration Biopsy

FNA should be performed for nodules meeting ACR TI-RADS criteria based on size and sonographic features 1.

FNA is particularly important for:

• Nodules >8-10 mm with suspicious ultrasound features 1
• Any nodule where preoperative diagnosis would change management 3

Core needle biopsy may be preferred over FNA when:

• The mass is likely not of thyroid origin (e.g., lymphoma, metastasis) 4
• FNA cytology is repeatedly non-diagnostic 3

When Biopsy Can Be Deferred

Pretreatment biopsy is not required if thyroid cancer diagnosis is highly probable and upfront surgical resection is achievable 1. However, this applies primarily to clearly resectable thyroid malignancies, not to initial workup of undifferentiated large masses 1.

Special Considerations

Distinguishing Thyroid from Non-Thyroid Masses

When evaluating an anterior mediastinal or lower neck mass, consider:

• Thymic tumors are more likely if the mass is in the thymic bed, tumor markers (AFP, beta-hCG) are negative, and there is no continuity with the thyroid gland 1
• Metastases to thyroid occur in 2.2% of thyroid malignancies, most commonly from lung, gastrointestinal, breast, and laryngeal primaries 4
• Previous cancer history should raise suspicion for metastatic disease 4

Compressive Symptoms

Thyroidectomy is indicated when a goiter causes compressive symptoms, regardless of thyroid function status 2. Document specific symptoms (dysphagia, dyspnea, positional symptoms) as these influence surgical decision-making 2.

Avoiding Common Pitfalls

• Do not rely on imaging alone without tissue diagnosis for large masses, as benign and malignant lesions overlap significantly in imaging characteristics 3, 5
• Do not skip ultrasound even if CT has already been performed, as US provides superior lymph node assessment 1
• Do not obtain radioiodine uptake scans for initial mass workup, as they do not help differentiate benign from malignant disease 1
• Incidental thyroid nodules have a 13.3% malignancy rate and require thorough workup, not dismissal 5