Table 1.

Main types of calcification found in thyroid lesions.

	Localization	Type of Lesion	Description
Microcalcification ≤1 mm
Psammoma bodies (PBs)	Inside lymph vessels or in the papillae axis	True PBs Classical PTC	50–70 μm round-shaped, concentrically laminated, calcified concretions with a glassy appearance (Figure 1C).
Inspissated colloid calcified	Inside follicles	False PBs Benign nodules	Thick colloid (colloid crystals) can present microcalcifications over inspissated colloid and lead to focal hyperechogenic foci; can potentially be confused with PBs (Figure 1B).
Stromal microcalcification	Around follicles	False PBs	Spherical crystalline bodies with a diameter of 0.1–2.5 μm. Usually too small to be detected by light microscopy and apparently arise within basal laminae as a result of concentric deposition of calcium salts or calcifications of membrane-bound vesicles. Calcified collagen fibrils can rarely be observed [27,28] (Figure 1A).
Bone calcification	Connective tissue	False PBs	Bone formation is considered when there is both bone matrix and osteocytes in the connective tissue of a thyroid nodule, regardless of being neoplastic or not [29,30]
Macrocalcification >1 mm
Eggshell, annular, or rim-like calcifications		Benign and malignant lesions	Annular or rim-like peripheral calcification, defined in US as curvilinear hyperechoic structures parallel to the margin of the nodule.
Coarse calcifications	Stroma	Benign and malignant lesions	An irregularly shaped focus of calcification (can comprise micro- and macrocalcifications) (Figure 1D).