Thyroid simple cyst is a completely cystic lesion in the thyroid gland, surrounded by a thin smooth wall and containing clear serous fluid. True thyroid cysts are rare, comprising only 1-3% of thyroid nodules; most 'cystic' thyroid lesions are actually degenerated colloid nodules. Simple cysts are classified as TR1 (benign) in the ACR TI-RADS classification and require no biopsy or follow-up. Usually asymptomatic and discovered incidentally. Large cysts may cause compressive symptoms.
Age Range
20-70
Peak Age
45
Gender
Female predominant
Prevalence
Common
Thyroid simple cysts develop from microcystic degeneration of follicular cells or from ultimobranchial body remnants (solid cell nests). In follicular-origin cysts, tubular obstruction or micro-hemorrhage leads to follicle expansion and cystic cavity formation. The content is clear serous fluid with low protein content, unlike colloid. The cyst wall is lined by thin single-layer cuboidal or flat epithelium — the thinness of this wall reflects the balance between internal and external pressure. On imaging, it appears completely anechoic because the fluid is homogeneous and there are no internal structures to reflect ultrasound waves. Comet-tail artifact (V-shaped echogenic artifact) originates from colloid crystals in the cyst wall and is an important finding supporting benignity.
The triad of completely anechoic (echo-free), round/oval lesion + prominent posterior acoustic enhancement + thin smooth wall is pathognomonic for simple cyst. The combination of these three findings meets ACR TI-RADS TR1 (benign) classification and requires no biopsy or follow-up.
A completely anechoic (echo-free), round or oval, well-defined cystic lesion is seen in the thyroid parenchyma. There is a thin smooth wall, and wall thickness is generally less than 1 mm. No echoes, septa, solid component, or debris within the lesion. Posterior acoustic enhancement is prominent and confirms pure fluid content. Surrounding thyroid parenchyma is normal. Per ACR TI-RADS v2017, this appearance is classified as TR1 (benign) and does not require biopsy.
Report Sentence
Simple cyst measuring ... mm in the [right/left] thyroid lobe, completely anechoic, well-defined, with posterior acoustic enhancement (ACR TI-RADS TR1). No biopsy or follow-up recommended.
A V-shaped echogenic focus (comet-tail artifact / reverberation artifact) may be seen at the cyst wall or immediately adjacent to it. This artifact originates from colloid crystals or microcalcifications and is a strong indicator favoring benignity. In ACR TI-RADS, comet-tail artifact scores 0 points and does not contribute to malignancy scoring. Unlike microcalcification, it shows V-shaped tailing.
Report Sentence
Comet-tail artifact (V-shaped echogenic focus) is seen at the cyst wall, a finding favoring benign colloid cyst/simple cyst.
No vascularity is seen within or on the wall of the cyst on color and power Doppler US — the lesion is completely avascular. Surrounding thyroid parenchyma shows normal vascularity. The avascular nature confirms the absence of solid component in the simple cyst. Detection of vascularity in the presence of solid component or thick septa excludes the diagnosis of simple cyst and requires evaluation as a mixed/complex lesion.
Report Sentence
No vascularity within or on the cyst wall on color and power Doppler examination, consistent with completely avascular simple cyst.
Non-contrast CT shows a homogeneous hypodense lesion at water density (0-20 HU) in the thyroid parenchyma. Thin smooth wall is present. No internal structure, calcification, or high-density area. Normal thyroid parenchyma is high density (80-120 HU) — the cyst stands out as markedly hypodense against this background. No wall or internal enhancement on contrast-enhanced series.
Report Sentence
Simple cyst measuring ... mm in the [right/left] thyroid lobe, at water density (... HU), homogeneous, well-defined, showing no enhancement.
On T2-weighted MRI, the simple cyst shows very marked hyperintense (bright) signal — similar signal characteristics to CSF due to free water content. Thin smooth wall, no internal structure, septa, or solid component. Appears hypointense (dark) on T1-weighted sequences — if proteinaceous content were present, relative T1 hyperintensity would be expected. No enhancement on gadolinium-enhanced series.
Report Sentence
Simple cyst in the [right/left] thyroid lobe, markedly hyperintense on T2, hypointense on T1, showing no enhancement.
On Tc-99m pertechnetate scintigraphy, a simple cyst appears as a 'cold' area with decreased uptake compared to surrounding parenchyma. Since the cyst consists of fluid content, there are no follicular cells or NIS activity. While the cold nodule finding is a scintigraphic finding that should also raise consideration of malignancy, a completely anechoic simple cyst appearance on US excludes malignancy.
Report Sentence
Photopenic (cold) area measuring ... mm in the [right/left] thyroid lobe on scintigraphy, requiring no further intervention when correlated with simple cyst findings on US.
Criteria
Arises from follicular epithelium or ultimobranchial body remnants. Lined by single-layer cuboidal or flat epithelium. Contains clear serous fluid.
Distinct Features
Completely anechoic on US, thin smooth wall. Aspirate is clear, thyroglobulin low. Cytology shows benign epithelial cells.
Criteria
Results from cystic degeneration of an existing colloid nodule. Completely cystic or predominantly cystic appearance. May contain dark yellow-brown viscous fluid. Most 'thyroid cysts' actually belong to this group.
Distinct Features
Sometimes accompanied by internal echoes (debris), comet-tail artifact, thick proteinaceous content, or small solid component. Aspirate may be dark brown/yellow 'chocolate' colored. Thyroglobulin is elevated.
Criteria
Results from hemorrhage into a cyst. In acute phase hyperechoic (high protein/fibrin) content, in subacute phase mixed echogenicity, in chronic phase clearing is seen. Usually presents with sudden painful swelling.
Distinct Features
On US, hyperechoic internal echoes, fibrin strands, fluid-debris level may be seen in the acute phase. T1 hyperintensity (methemoglobin) on MRI is diagnostic. Shows spontaneous resolution over time.
Distinguishing Feature
Colloid nodule is typically characterized by isoechoic or hyperechoic internal echoes, comet-tail artifacts and spongiform pattern. Unlike a completely anechoic simple cyst, it may contain internal echoes or solid component. Aspirate is thick colloidal fluid.
Distinguishing Feature
Cystic variant of papillary carcinoma may show predominantly cystic appearance but the presence of eccentric solid component (mural nodule), microcalcifications and vascularized solid area is distinguishing. Simple cyst has no solid component.
Distinguishing Feature
Cystic parathyroid adenoma may present as an anechoic/hypoechoic lesion posterior to the thyroid and can mimic a thyroid simple cyst. Distinguishing features: extrathyroidal location (outside thyroid capsule, posterior), polar vessel sign, and hypercalcemia/elevated PTH.
Distinguishing Feature
Thyroglossal duct cyst is usually located in the midline (at or above the isthmus level) and is seen adjacent to the thyroid rather than within the thyroid tissue. It moves with swallowing and has anatomical connection to the tongue. Thyroid simple cyst is within the gland parenchyma.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upThyroid simple cyst is classified as ACR TI-RADS TR1 (benign) and requires no biopsy or routine follow-up. Large cysts (>4 cm) may cause compressive symptoms (dysphagia, dyspnea) — in such cases, US-guided aspiration or sclerotherapy (ethanol injection) may be performed. Surgery (lobectomy) is rarely needed for recurrent symptomatic cysts. Cytological evaluation of aspirate fluid is recommended when atypical cells are present (to exclude cystic variant of papillary carcinoma) but is not routinely needed for completely anechoic simple cysts.
Simple thyroid cysts are benign and require no treatment (ACR TI-RADS TR1). Aspiration or surgery may be considered for large cysts with compressive symptoms. Recurrence after aspiration is common. If solid component is detected, malignancy should be excluded.