When is a thyroid ultrasound recommended?

When to Order Thyroid Ultrasound

Thyroid ultrasound is the preferred first-line imaging modality for evaluating thyroid nodules in euthyroid patients and should be ordered when there is a palpable thyroid abnormality, incidental nodule detection, high-risk clinical features (radiation exposure, family history), or to characterize nodules for malignancy risk stratification. 1, 2

Initial Diagnostic Pathway

Start with thyroid function tests (TSH) before ordering any imaging, as the TSH result determines the appropriate imaging pathway. 2

• For normal TSH (euthyroid state): Thyroid ultrasound is the preferred first-line imaging modality, providing superior morphological evaluation of thyroid nodules and cervical lymph nodes. 3, 2
• For low TSH (thyrotoxicosis): Perform thyroid ultrasound first to evaluate morphology, followed by radioiodine uptake scan to determine the cause of thyrotoxicosis. 2
• For high TSH (hypothyroidism): Imaging is generally not indicated, but if needed for a palpable abnormality, ultrasound is appropriate. 2

Specific Clinical Indications for Thyroid Ultrasound

Palpable Thyroid Abnormalities

• Any palpable thyroid nodule warrants ultrasound evaluation, as physical examination alone misses 45% of additional nodules and fails to detect nodules in 16% of patients with presumed solitary palpable nodules. 4
• Ultrasound detects nodules in approximately 18% of patients with clinically diagnosed multinodular goiter who have no nodules >1 cm on imaging. 4

High-Risk Populations Requiring Screening

• History of head and neck radiation exposure: These patients should undergo screening thyroid ultrasound, as radiation exposure significantly increases thyroid cancer risk and ultrasound detects several times more cases than palpation alone (150 additional cancers detected per 10,000 screened patients). 5, 6, 4
• Family history of thyroid cancer: Screening ultrasound is recommended to lower the threshold for early detection. 1, 4
• Age <15 years with thyroid abnormality: Younger age increases baseline malignancy probability. 1

Symptomatic Presentations

• Compressive symptoms (dyspnea, orthopnea, dysphagia, dysphonia): Ultrasound confirms thyroid origin and characterizes morphology in suspected goiter with obstructive symptoms. 2
• Rapidly growing nodule: Suggests aggressive biology requiring immediate ultrasound characterization. 1
• Vocal cord paralysis: Indicates possible invasive disease requiring urgent imaging. 1

Thyroiditis Evaluation

• Palpable thyroid nodule in the setting of thyroiditis: Ultrasound is indicated when there is concern for coexisting thyroid malignancy. 2
• Atypical presentation of thyroiditis: Ultrasound helps differentiate causes, though most straightforward cases can be diagnosed clinically with laboratory confirmation alone. 2
• Doppler ultrasound can differentiate overactive thyroid from destructive thyroiditis with 95% sensitivity and 90% specificity. 2

Size Thresholds and Risk Stratification

General Population

• Thyroid nodules >1 cm detected on ultrasound are present in 2-5% of the population with normal thyroid examination. 4
• Any nodule >1 cm with suspicious ultrasound features should undergo FNA, regardless of other characteristics. 1

Size-Based Malignancy Risk (Important Nuance)

• Contrary to common assumptions, larger nodules have LOWER malignancy rates: Nodules <2 cm have the highest malignancy rate (30%), while nodules ≥2 cm have similar risk (20%). 7
• Among nodules undergoing FNA, malignancy rates decline stepwise with increasing size: 57% for nodules <1 cm to 20% for nodules >6 cm. 7
• Size alone is a poor predictor of malignancy and should not be used as an independent risk factor; the constellation of ultrasound features is more important. 7, 8

Ultrasound Features That Trigger Further Evaluation

Once ultrasound is performed, the following features indicate need for FNA:

• Microcalcifications: Highly specific for papillary thyroid carcinoma (OR 6.4). 1, 8
• Marked hypoechogenicity: Solid nodules darker than surrounding thyroid parenchyma. 1, 8
• Irregular or microlobulated margins: Infiltrative borders rather than smooth contours (OR 4.8). 1, 8
• Absence of peripheral halo: Loss of thin hypoechoic rim normally surrounding benign nodules. 1, 8
• Central hypervascularity: Chaotic internal vascular pattern. 1
• Solid composition: Higher malignancy risk compared to cystic nodules. 1

Common Pitfalls to Avoid

• Do not proceed directly to radioiodine uptake scan in euthyroid patients: This wastes resources and has low diagnostic value for determining malignancy. 2
• Do not rely on palpation alone for screening high-risk patients: Palpation sensitivity for all-sized nodules is only 10-41%, missing the majority of nodules detected by ultrasound. 6
• Do not use ultrasound for routine hypothyroidism workup in adults: It does not differentiate causes of hypothyroidism. 2
• Do not assume larger nodules are more concerning: The inverse relationship exists, with smaller nodules having higher malignancy rates. 7
• Do not perform screening ultrasound without specific clinical indication: Population screening is not recommended by WHO given the low overall thyroid cancer rate (3-5%) and favorable prognosis of most papillary carcinomas. 3

Special Considerations for Small Nodules (<1 cm)

• TIRADS guidelines generally recommend surveillance rather than FNA for nodules <1 cm unless high-risk features are present (subcapsular location, suspicious lymphadenopathy, radiation history, family history). 3, 1
• Small nodules have similar histological aggressiveness as larger nodules when malignant, but lower clinical significance for relapse. 3, 8
• Ultrasound resolution has improved dramatically over time, leading to detection of many small nodules that may not require immediate intervention. 5

Follow-Up Ultrasound Considerations

• Patients on thyroid hormone suppression therapy: Nodule volume tends to decrease (by 0.20 cm³), while nodules in untreated patients increase (by 0.34 cm³). 5
• Serial ultrasounds in radiation-exposed patients: Useful for identifying growing lesions that warrant FNA, as FNA of all nodules is not feasible in this high-prevalence population. 5