Parathyroid ultrasound is a localization study, not a diagnostic one. The diagnosis of primary hyperparathyroidism is biochemical — elevated calcium with an inappropriately elevated parathyroid hormone. By the time the patient reaches you, the question is not whether there is a parathyroid problem but where the culprit gland is, so the surgeon can plan a targeted, minimally invasive parathyroidectomy rather than a four-gland exploration.

That framing changes how you scan. You are hunting for a specific small structure in a set of predictable locations.

Normal Anatomy

Most people have four parathyroid glands, though the number varies — supernumerary glands occur in a meaningful minority.

Superior glands (from the fourth branchial pouch): More consistent in position. Typically posterior to the mid-to-upper thyroid lobe, near the cricothyroid junction, close to the recurrent laryngeal nerve and inferior thyroid artery.
Inferior glands (from the third branchial pouch): Much more variable, because they descend a longer embryological distance with the thymus. Usually near the lower thyroid pole, but they can end up anywhere along that descent path.

Normal parathyroid glands are typically not visible on ultrasound. They are small — around 5 × 3 × 1 mm — and isoechoic with surrounding fat. If you can see it, it is probably abnormal. This is genuinely useful: the study is not about identifying four normal glands, it is about spotting the one that has enlarged.

Technique

Probe: High-frequency linear, 10–15 MHz. Drop to a lower frequency for deep or retrosternal assessment.

Position: Supine with the neck extended over a pillow or roll — the same setup as thyroid scanning. Full extension brings low-lying glands up into view.

Approach: Scan the thyroid first and completely. You need to know the thyroid anatomy, identify any nodules, and be able to say confidently that a lesion is outside the thyroid capsule.

Then sweep systematically:

Transverse through the entire thyroid from above the upper pole to below the lower pole, watching the posterior aspect
Longitudinal through both lobes
Extend the search superiorly along the carotid sheath, inferiorly toward the thoracic inlet, and posteriorly toward the tracheoesophageal groove
Swallowing manoeuvre — asking the patient to swallow can bring a low-lying gland up into view transiently

Sonographic Features of a Parathyroid Adenomas

Echogenicity: Characteristically hypoechoic — usually markedly so, notably darker than adjacent thyroid tissue. This is the most consistent feature.

Shape: Oval or oblong, often flattened, with the long axis typically oriented craniocaudally. A well-defined, smooth margin.

Location: Posterior to the thyroid, separated from it by a hyperechoic plane representing the thyroid capsule and intervening fascia. Demonstrating that plane is the key to proving the lesion is extrathyroidal.

Size: Most adenomas are 1–3 cm. Larger lesions correlate loosely with higher PTH levels.

Vascularity — the polar vessel sign: A prominent feeding artery entering at one pole of the lesion, usually a branch of the inferior thyroid artery, which then arborizes around the periphery. This is the most specific colour Doppler feature and is worth actively looking for. Internal vascularity is generally increased compared with a lymph node.

Cystic change and calcification occur but are less common; a predominantly cystic parathyroid lesion is unusual.

The Key Differentials

Getting this right is most of the skill in the study.

Parathyroid Adenoma vs Lymph Node

Feature	Adenoma	Lymph node
Echogenic hilum	Absent	Present in benign nodes
Shape	Oval, oblong	Reniform
Vascularity	Polar feeding vessel, peripheral arborization	Hilar branching pattern
Echogenicity	Markedly hypoechoic	Hypoechoic cortex, echogenic hilum

The echogenic hilum is the single most useful discriminator. A benign lymph node has one; a parathyroid adenoma does not.

Parathyroid Adenoma vs Thyroid Nodule

The critical question is whether the lesion is inside or outside the thyroid capsule.

Look for the hyperechoic separating plane between lesion and thyroid
Ask the patient to swallow and watch: a thyroid nodule moves with the thyroid as a single unit; an extrathyroidal parathyroid adenoma may move independently or slide relative to the gland
Scan in both planes — a posterior exophytic thyroid nodule can mimic an adenoma convincingly in one plane and reveal its thyroid origin in the other

Intrathyroidal parathyroid adenomas exist and are a genuine diagnostic trap. They are a recognized cause of failed initial parathyroidectomy. If a patient has clear biochemical hyperparathyroidism and no adenoma is found in the usual locations, consider an intrathyroidal gland and say so in the report.

Other Mimics

Oesophagus — a classic pitfall on the left, in the tracheoesophageal groove. It has a gut signature and target appearance in transverse. Ask the patient to swallow and watch it move and change shape.
Longus colli muscle — deep and posterior, but has muscle striations and does not have a polar vessel
Thyroid nodule of the posterior lobe

Ectopic Locations

Roughly one in six or so parathyroid adenomas sits somewhere other than the expected position — a common reason for a negative ultrasound in a patient with unmistakable biochemical disease.

Where to extend the search:

Intrathyroidal
Retro-oesophageal / tracheoesophageal groove
Carotid sheath
Undescended, high in the neck near the carotid bifurcation — for superior glands
Thymic / mediastinal — inferior glands that descended too far. Ultrasound cannot see below the thoracic inlet, which is a hard limitation.

Report the limitation explicitly. "No adenoma identified within the sonographically accessible neck; a mediastinal or retro-oesophageal location cannot be excluded by ultrasound" is a far more useful report than "no adenoma seen."

Multi-Gland Disease

Most primary hyperparathyroidism is caused by a single adenoma, which is why targeted surgery works. But:

Double adenomas occur in a small percentage
Four-gland hyperplasia is characteristic of secondary hyperparathyroidism in chronic kidney disease and of familial syndromes such as MEN
Parathyroid carcinoma is rare. Suspicious features include large size, irregular or infiltrative margins, marked hypoechogenicity, and invasion of adjacent structures — usually accompanied by strikingly high calcium and PTH.

Do not stop after finding one lesion. Complete the survey of all four expected positions. Finding a second enlarged gland changes the operation from a targeted excision to a bilateral exploration.

Complementary Imaging

Ultrasound is rarely used alone for surgical planning.

Sestamibi scintigraphy (with SPECT/CT) — functional imaging, better for ectopic and mediastinal glands
4D-CT — increasingly used, excellent anatomical localization, particularly in reoperative cases

Concordance between ultrasound and sestamibi substantially increases confidence in a targeted approach. When the two disagree, or when both are negative, the surgeon generally plans a bilateral exploration. Your report contributes to that decision, so be clear about your degree of confidence.

Intraoperative PTH monitoring confirms cure during surgery — a rapid fall after excision indicates the offending gland has been removed.

Key Images to Capture

Full thyroid survey — transverse and longitudinal, both lobes
Any candidate lesion in two planes with three measurements
The hyperechoic plane separating the lesion from the thyroid
Colour Doppler demonstrating the polar feeding vessel
Relationship to the thyroid lobe, carotid, and trachea, described precisely
Documentation that all four expected gland positions were surveyed
Any cervical lymphadenopathy
Coexisting thyroid nodules — these matter for operative planning

Reporting Language

Positive: "Posterior to the lower pole of the right thyroid lobe there is a 1.6 × 0.9 × 0.7 cm well-defined markedly hypoechoic oval lesion, separated from the thyroid by an intact hyperechoic plane. Colour Doppler demonstrates a prominent polar feeding vessel with peripheral arborization. No echogenic hilum. The remaining three expected gland positions were surveyed and no further enlarged gland identified. Appearances are highly suggestive of a right inferior parathyroid adenoma. Correlation with sestamibi imaging and biochemistry is recommended."

Negative: "No enlarged parathyroid gland identified in the expected superior or inferior positions bilaterally. The tracheoesophageal grooves and carotid sheaths were examined and are unremarkable. Note that ultrasound cannot assess below the thoracic inlet; a mediastinal or retro-oesophageal adenoma cannot be excluded. In the context of confirmed biochemical hyperparathyroidism, cross-sectional or functional imaging is recommended."

SonoBuddy is a reference tool for sonographers — localization findings support, but do not replace, surgical and biochemical assessment.

Parathyroid Ultrasound: Localizing Adenomas Before Surgery