Thyroid nodules on ultrasound

What it is

Thyroid nodules are very common: many adults have one on ultrasound, and almost all are benign. Ultrasound is the imaging test for every known or suspected nodule. It describes each nodule by five features: composition (cystic, spongiform, mixed or solid), echogenicity (brighter, the same or darker than normal thyroid, or darker than the neck muscles), shape (wider-than-tall or taller-than-wide on a transverse view), margin (smooth, lobulated, irregular or spreading outside the gland) and bright spots inside (echogenic foci such as comet-tail artifacts or microcalcifications). Risk-stratification systems such as TI-RADS combine these features into a category from benign to highly suspicious, and the category together with the nodule's size decides whether to sample it with fine-needle aspiration (FNA), follow it with repeat scans or leave it. The scan also checks the lymph nodes of the neck. Before any of this, a TSH tells whether the nodule may be hyperfunctioning.

What it looks like

• Reassuring: a purely cystic nodule, a spongiform nodule (many tiny cysts making up most of it) and comet-tail artifacts from colloid.
• Suspicious: solid, markedly hypoechoic (darker than the strap muscles), taller-than-wide, irregular or infiltrative margin.
• Microcalcifications (tiny bright dots without shadows) are a classic sign of papillary carcinoma.
• Extrathyroidal extension, an interrupted rim calcification with soft tissue bulging through it, and suspicious neck nodes raise the risk sharply.
• TI-RADS adds points for each feature to give a category; higher categories need FNA at a smaller size.
• Abnormal neck node: round, lost fatty hilum, microcalcifications, cystic change or chaotic peripheral vessels.

Why it looks that way

• Malignant cells pack tightly with little colloid, so they reflect less sound and look darker than normal thyroid.
• A taller-than-wide shape means the nodule grows across tissue planes rather than pushing them aside.
• Colloid crystals cause the small comet-tail reverberation seen in benign colloid nodules.

Differential and pitfalls

• Diffuse thyroid disease can mimic nodules: Hashimoto thyroiditis gives a coarse, darker gland with pseudonodules.
• Graves disease shows a diffusely enlarged gland with very increased flow on colour Doppler.
• Large comet-tail artifacts are benign, but small bright dots without a tail may be microcalcifications: the radiologist decides.
• A low TSH changes the path: a hot nodule on scintigraphy is rarely malignant and usually needs no FNA.
• FNA cytology is reported with the Bethesda system (six categories), from non-diagnostic to malignant.

Next step

• Any thyroid nodule: request TSH first, then a thyroid and neck ultrasound reported with a risk category.
• Low TSH: request thyroid scintigraphy; a hot nodule is treated as hyperfunctioning, not biopsied.
• High-risk features above the size threshold, or a suspicious neck node: refer for ultrasound-guided FNA.
• Low-risk nodule below the threshold: follow-up ultrasound at the interval the report suggests.
• Rapid growth, hoarseness, a hard fixed mass or stridor: urgent referral to head and neck surgery or endocrinology.

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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: Thyroid Nodule (StatPearls (NCBI Bookshelf), 2023); Thyroid Nodule Biopsy (StatPearls (NCBI Bookshelf), 2023).

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