KEY FACTS
Imaging
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Typically, unilocular, thin-walled cyst
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Anechoic content with posterior acoustic enhancement
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Echogenic foci with comet-tail artifacts are characteristic
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Represent suspended colloid aggregates
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Hemorrhage into cyst results in thick wall with debris, septa, or fluid level
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Colloid cyst can arise in hyperplastic nodule
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Background solid, isoechoic nodule with colloid aggregates in cystic spaces of variable sizes; no punctate calcification
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Ultrasound-guided fine-needle aspiration is not necessary for diagnosis
Top Differential Diagnoses
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Thyroid adenoma
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Simple thyroid cyst
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Differentiated thyroid carcinoma
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Thyroglossal duct cyst
Pathology
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Benign lesion without malignant potential
Clinical Issues
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Common (15-25% of thyroid nodules)
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Most are incidentally detected on ultrasound
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May present as palpable nodule when large or as rapidly enlarging nodule if bleeding occurs into cyst
Scanning Tips
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Colloid crystals may mimic echogenic foci from punctate calcification seen in papillary thyroid carcinoma
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Compound imaging may show comet-tail artifacts behind punctate echogenic focus
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Scanning in fundamental mode helps to distinguish comet tail from posterior shadowing
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Beware of cystic malignancy if cystic lesion contains eccentric solid nodule with color flow or if there are pathologic lymph nodes
. These represent colloid aggregates in viscous fluid containing thyroglobulin.
.
. The comet-tail artifact is not V-shaped.
and avascular debris 
with little colloid 
.
