What are pseudonodules on thyroid ultrasound?

Thyroid Pseudonodules on Ultrasound

Pseudonodules are focal areas within the thyroid gland that mimic true nodules on ultrasound but are actually manifestations of underlying thyroid parenchymal disease—most commonly Hashimoto's thyroiditis—rather than discrete neoplastic lesions. These represent heterogeneous areas of inflammation, fibrosis, or focal lymphocytic infiltration that create the false appearance of a nodule 1, 2, 3.

Key Ultrasound Characteristics

In Hashimoto's Thyroiditis Context

The most common pseudonodules occur in Hashimoto's thyroiditis and typically demonstrate:

• Marked hypoechogenicity (most common finding)
• Smooth, circumscribed margins (not irregular or spiculated)
• Ovoid/horizontal orientation (not taller-than-wide)
• Absence of calcifications (no microcalcifications or macrocalcifications)
• Vascularity similar to adjacent parenchyma (not increased central or peripheral vascularity)
• Ill-defined borders that blend with heterogeneous thyroid parenchyma 4, 3

Distinguishing Features from True Nodules

The critical differentiating feature is that pseudonodules lack the suspicious ultrasound characteristics associated with malignancy 3. According to the 2020 C-TIRADS guidelines, pseudonodules in Hashimoto's thyroiditis should be recognized and potentially downgraded in risk category when clinical history confirms thyroiditis 1.

The ACR TI-RADS specifically notes that certain benign patterns in Hashimoto's thyroiditis—including variegated hyperechoic areas separated by hypoechoic bands (described as "giraffe hide" pattern)—represent pseudonodular changes rather than true nodules 4.

Clinical Context Matters

Subacute Thyroiditis

Pseudonodules can also occur in subacute thyroiditis, appearing as:

• Nodules lacking spherical shape
• Ill-defined margins
• Often multifocal distribution

When these features are present, ask specifically about recent cold/flu symptoms or neck pain 1. If confirmed, classify as lower risk (C-TIRADS 2-3) and pursue short-term follow-up rather than immediate biopsy. If nodules shrink on follow-up, this confirms the pseudonodular nature 1.

Non-Thyroidal Mimics

Pharyngoesophageal Diverticulum

A critical pitfall: pharyngoesophageal diverticula can masquerade as posterior thyroid nodules on ultrasound 5, 6. Key distinguishing features include:

• Hyper-hypo-hyper-echogenic wall pattern (diverticulum wall structure)
• Continuity with esophageal wall on multiple scanning planes
• Gas-like hyperechogenicity with changing tail patterns during swallowing
• Anechoic fluid content in liquid-containing diverticula
• Location posterior to thyroid 6

Always scan multiple planes and observe during swallowing to identify continuity with the esophagus 6.

Diagnostic Approach

When to Suspect Pseudonodules

1. Patient has known or suspected Hashimoto's thyroiditis (diffusely heterogeneous gland)
2. Recent history of viral illness or neck pain (subacute thyroiditis)
3. Marked hypoechogenicity WITHOUT other suspicious features (no microcalcifications, irregular margins, or taller-than-wide shape)
4. Multiple similar lesions in heterogeneous parenchyma

Role of Fine Needle Aspiration

While FNA remains the gold standard for definitively distinguishing pseudonodules from true nodules 2, 7, sonoelastography can non-invasively differentiate pseudonodules from true nodules with high accuracy 2. This may eliminate unnecessary biopsies in appropriate clinical contexts.

For inflammatory pseudonodules, cytology shows variable lymphocytic infiltration (mild 28.6%, moderate 39.2%, severe 32.1%) without features of true neoplasia 3.

Common Pitfalls to Avoid

1. Don't automatically biopsy marked hypoechoic nodules in Hashimoto's thyroiditis—assess for other suspicious features first 3

2. Don't miss posterior pharyngoesophageal diverticula—always evaluate continuity with esophagus and observe during swallowing 5, 6

3. Don't ignore clinical history—recent viral illness or neck pain should prompt consideration of subacute thyroiditis with pseudonodular changes 1

4. Don't assign points for ill-defined margins in heterogeneous glands—the ACR TI-RADS specifically states to assign zero points when margins cannot be determined due to heterogeneous parenchyma 4