23.1 Thyroid & Parathyroid

Scanning Technique

The thyroid and parathyroid are superficial structures, so they are imaged with a high-frequency linear-array transducer, typically 7.5–15 MHz, to maximize spatial resolution at depths of only 1–4 cm. The patient is positioned supine with the neck hyperextended, usually by placing a rolled towel or pillow beneath the shoulders so the chin points toward the ceiling. This extension moves the gland superiorly out of the thoracic inlet and closer to the skin surface. Both lobes are surveyed in transverse and sagittal (longitudinal) planes, and the isthmus — the thin band of tissue connecting the lobes anterior to the trachea — is measured transversely. Side-by-side comparison images of the two lobes reveal subtle asymmetry.

Key landmarks on a transverse image through the mid-neck:

• Thyroid lobe — the homogeneous structure anterolateral to the trachea
• Trachea — midline, casting a strong acoustic shadow from intraluminal air
• Common carotid artery — round, pulsatile, anechoic structure lateral to the lobe
• Internal jugular vein — oval, compressible, anterolateral to the carotid
• Longus colli muscle — hypoechoic, posterior to each lobe along the spine
• Strap muscles — hypoechoic bands anterior to the gland

Normal Thyroid Appearance and Size

Normal thyroid parenchyma is homogeneous with a fine, medium-level echotexture that is hyperechoic relative to the strap muscles and roughly isoechoic to the submandibular gland. An adult lobe measures approximately 4–6 cm in length, 1.3–1.8 cm in width, and 1.5–2 cm in anteroposterior (AP) dimension; an AP measurement above 2 cm suggests enlargement. The isthmus normally measures only a few millimeters in AP thickness (normally under about 3 mm (and always under 5 mm)). Volume is calculated with the prolate ellipse formula: length × width × AP × 0.523 per lobe, with normal total volume roughly 10–15 mL (slightly larger in men). Color Doppler shows modest, evenly distributed parenchymal flow.

Diffuse Thyroid Disease

Graves disease is an autoimmune hyperthyroidism. The gland is diffusely enlarged and hypoechoic, and color Doppler demonstrates markedly increased vascularity — the classic "thyroid inferno" — with elevated peak systolic velocities in the inferior thyroidal arteries. Patients have suppressed thyroid-stimulating hormone (TSH) with elevated T3/T4.

Hashimoto thyroiditis (chronic lymphocytic thyroiditis) is the most common cause of hypothyroidism in iodine-sufficient countries. Sonographically the gland becomes heterogeneous, hypoechoic, and micronodular, often with echogenic fibrous septations producing a giraffe-like or pseudonodular pattern. Early disease may be hypervascular, but the end-stage gland is small, fibrotic, and atrophic. Patients may have elevated TSH and positive antithyroid antibodies.

Subacute (de Quervain) thyroiditis appears as patchy, ill-defined hypoechoic regions with decreased flow in the affected areas, typically following a viral illness with a tender gland.

Nodule Evaluation

Thyroid nodules are extremely common; the sonographer's task is to document features that stratify malignancy risk. The Thyroid Imaging Reporting and Data System (TI-RADS) assigns points across five feature categories, and the total determines whether fine-needle aspiration (FNA) or follow-up is recommended.

A purely cystic nodule with comet-tail artifact — bright echogenic foci with tapering reverberation tails — represents colloid crystals and is essentially always benign. Spongiform (honeycomb) nodules are likewise benign. Taller-than-wide shape is among the most specific single predictors of malignancy because malignant nodules grow against tissue planes rather than along them.

Thyroid Malignancies

• Papillary carcinoma — by far the most common thyroid malignancy (roughly 80% of cases). Sonographic hallmarks include a solid hypoechoic nodule with punctate microcalcifications (psammoma bodies), irregular margins, and taller-than-wide shape. It spreads to regional lymph nodes, which may show microcalcifications or cystic change. Prognosis is excellent.
• Follicular carcinoma — typically a solid, isoechoic nodule whose malignancy is determined by capsular or vascular invasion at pathology; it spreads hematogenously.
• Medullary carcinoma — arises from parafollicular C cells and produces calcitonin, so serum calcitonin is elevated. Associated with multiple endocrine neoplasia (MEN) type 2. Nodules often contain coarse calcifications.
• Anaplastic carcinoma — a rapidly growing, large, invasive mass of older adults that frequently compresses or invades the trachea, esophagus, and vessels; prognosis is poor.

A multinodular goiter is an enlarged gland containing multiple nodules of varying echogenicity, often with cystic degeneration, coarse calcification, and comet-tail colloid. Each nodule is still assessed individually — the most suspicious-looking one (not necessarily the largest) is selected for biopsy.

Parathyroid Imaging

Normal parathyroid glands (usually four) are only a few millimeters and are not routinely seen. A parathyroid adenoma — the most common cause of primary hyperparathyroidism — appears as an oval, homogeneous, markedly hypoechoic solid mass posterior to the thyroid lobe, often near the inferior pole, and is typically hypervascular with a peripheral feeding vessel. The clinical clue is the laboratory pair of elevated serum calcium and elevated parathyroid hormone (PTH). Adenomas must be distinguished from exophytic thyroid nodules and from lymph nodes (nodes have a fatty hilum). Parathyroid hyperplasia enlarges multiple glands, and carcinoma is rare and usually larger and more irregular.

Ultrasound-Guided FNA Basics

Ultrasound guides fine-needle aspiration by providing real-time visualization of the needle tip within the nodule, avoiding the carotid artery and jugular vein. Indications are driven by the ACR TI-RADS level and a size threshold that falls as suspicion rises. Biopsy (FNA) thresholds: TR3 ≥2.5 cm, TR4 ≥1.5 cm, TR5 ≥1.0 cm. Sonographic follow-up thresholds sit one step lower: TR3 ≥1.5 cm, TR4 ≥1.0 cm, TR5 ≥0.5 cm. TR1 (benign) and TR2 (not suspicious) require neither.