What are the differentials for a sudden increase in thyroid mass?

Differential Diagnosis of Sudden Increase in Thyroid Mass

The most critical differential for a sudden increase in thyroid mass is hemorrhage into a pre-existing nodule or cyst, followed by rapidly progressive lymphoma (particularly Burkitt's lymphoma) and anaplastic thyroid carcinoma, all of which require urgent evaluation to prevent life-threatening airway compromise.

Life-Threatening Differentials Requiring Immediate Action

Hemorrhage into Thyroid Nodule/Cyst

• Most common cause of acute thyroid enlargement, occurring when bleeding develops within a pre-existing benign nodule or cystic lesion 1
• Presents with sudden onset of neck pain, swelling, and potential compressive symptoms 1
• Can occur spontaneously in solid or cystic nodules, though cystic nodules are more prone to this complication 1
• Immediate ultrasound is the first-line imaging to characterize the mass and assess for hemorrhagic changes 2, 3

Rapidly Progressive Lymphoma

• Burkitt's lymphoma can present as a rapidly expanding thyroid mass developing over days, causing tracheal compression within hours 4
• Requires emergent fine-needle aspiration (FNA) to establish diagnosis, showing monotonous intermediate-sized lymphoid cells 4
• CT neck with contrast should be performed urgently to assess airway compromise and extent of disease 4
• Can progress to life-threatening airway obstruction within 12-24 hours if untreated 4
• Other thyroid lymphomas typically occur in patients with pre-existing Hashimoto's thyroiditis 5

Anaplastic Thyroid Carcinoma

• Presents as a rapidly enlarging, hard, fixed thyroid mass with invasion into surrounding structures 2
• Accounts for approximately 2% of thyroid cancers but carries extremely poor prognosis 2
• Often causes compressive symptoms including dysphagia, dyspnea, and hoarseness due to aggressive local invasion 2
• CT neck with contrast is essential to evaluate for aerodigestive tract invasion 2

Urgent Differentials Requiring Prompt Evaluation

Acute Thyroiditis

• Subacute (de Quervain's) thyroiditis can cause rapid thyroid enlargement with severe pain, fever, and transient hyperthyroidism 6
• Suppurative thyroiditis (rare) presents with acute painful swelling, fever, and systemic toxicity
• Diagnosis supported by elevated inflammatory markers and thyroid function tests showing initial thyrotoxicosis 6

Rapidly Growing Malignancy

• Poorly differentiated thyroid carcinoma can demonstrate accelerated growth patterns 2
• Medullary thyroid carcinoma occasionally presents with rapid enlargement 2
• Metastatic disease to the thyroid (renal cell carcinoma, melanoma, lung cancer) can cause sudden mass effect 2

Diagnostic Algorithm for Sudden Thyroid Mass Increase

Immediate Assessment (Within Hours)

1. Evaluate airway patency - assess for stridor, dyspnea, dysphagia, or voice changes indicating tracheal compression 4
2. Urgent thyroid ultrasound to characterize the mass, assess for hemorrhage, and evaluate cervical lymph nodes 2, 3
3. If airway compromise present: Obtain emergent CT neck with contrast to define extent and plan intervention 2, 4

Rapid Diagnostic Workup (Within 24-48 Hours)

1. Ultrasound-guided FNA for any solid or suspicious components, even if hemorrhage is present 3

   • Cytology can distinguish lymphoma, anaplastic carcinoma, and other malignancies from benign hemorrhage 4
   • Pathology review by experienced thyroid pathologist is essential 5

2. Laboratory evaluation:

   • TSH and free T4 to assess thyroid function 2, 6
   • Complete blood count and inflammatory markers if infection/thyroiditis suspected 6
   • Serum calcitonin if medullary thyroid cancer is a consideration 3

3. CT neck with contrast (if not already performed) to evaluate:

   • Tumor vascular encasement and invasion 2
   • Central and lateral neck lymphadenopathy 2
   • Mediastinal extension 2

Critical Pitfalls to Avoid

• Never assume growth equals malignancy alone: Most benign solid thyroid nodules grow over time, with 89% showing volume increase after 5 years 1
• Do not delay FNA in rapidly growing masses - waiting for "observation" can be catastrophic in lymphoma or anaplastic carcinoma 4
• Recognize that hemorrhage can obscure underlying pathology: Reaspirate after hemorrhage resolves if initial cytology is non-diagnostic 1
• Inflammatory background in Hashimoto's thyroiditis can complicate cytological interpretation and increase lymphoma risk 5
• Multiple suspicious ultrasound features together (hypoechogenicity, microcalcifications, irregular borders, solid composition, abnormal vascularity) substantially increase malignancy risk beyond any single feature 2, 3

Additional Rare Considerations

• Thyrolipoma: Rare benign tumor with substantial adipose tissue growth, typically presents as slowly growing mass but can occasionally enlarge more rapidly 7
• Toxic nodular goiter with acute thyrotoxicosis: Autonomous hyperfunctioning nodules can cause sudden symptoms, though mass effect is usually gradual 6