Parathyroid adenoma is the most common neoplasm of the parathyroid gland and is responsible for 80-85% of primary hyperparathyroidism. It is typically a solitary, well-defined, hypovascular or hypervascular lesion located posterior to the thyroid gland (between the posterior capsule and longus colli muscle). Average size is 10-20 mm. It is 3-4 times more common in women and peaks between ages 50-60. Clinically presents with hypercalcemia, renal stones, osteoporosis, and neuropsychiatric symptoms (bones, stones, moans, groans). On US, it appears as a hypoechoic solid lesion posterior to the thyroid, and the 'polar vessel sign' on Doppler is characteristic. Tc-99m sestamibi scintigraphy with dual-phase study and 4D-CT are gold standards for localization.
Age Range
40-70
Peak Age
55
Gender
Female predominant
Prevalence
Uncommon
Parathyroid adenoma develops from monoclonal proliferation of chief cells or oxyphil cells of the parathyroid gland. Adenomatous cells have escaped control of the normal calcium-PTH feedback mechanism — the calcium-sensing receptor (CaSR) set point is elevated or inactivated, so PTH secretion continues despite high calcium levels. Excessive PTH increases bone resorption (hypercalcemia), increases calcium reabsorption in the kidney, and stimulates 1,25-dihydroxy vitamin D synthesis. On imaging, it appears as a hypoechoic solid lesion because adenomatous parathyroid tissue has a cellular structure with lower acoustic impedance than thyroid parenchyma (colloid-rich, high iodine). On sestamibi scintigraphy, it shows late retention because mitochondrial count is increased in adenomatous cells — sestamibi is a lipophilic radiopharmaceutical that binds to active mitochondria and is retained for long periods in parathyroid adenomas with high oxidative metabolism.
The appearance of a prominent feeding artery entering the parathyroid adenoma from one pole (usually inferior) on color Doppler US. It is a branch of the inferior thyroid artery and is the main vascular source of the adenoma. A finding highly specific to parathyroid adenoma, different from the peripheral vascularity pattern of thyroid nodules or the hilar vascularity pattern of lymph nodes.
A hypoechoic, homogeneous, well-defined solid lesion is seen immediately posterior to the thyroid capsule, between the thyroid and longus colli muscle (prevertebral muscle). Usually oval or bean-shaped. Size varies between 10-30 mm (average 15 mm). Markedly more hypoechoic than thyroid parenchyma — echogenicity approaches muscle level. A thin hyperechoic capsule may be seen. In ectopic adenomas (intrathyroid, carotid sheath, mediastinum, retroesophageal), atypical location is observed. US sensitivity ranges 75-90% and is operator-dependent.
Report Sentence
Well-defined, hypoechoic, homogeneous solid lesion measuring ... x ... mm immediately posterior to the [right/left] thyroid lobe capsule, consistent with parathyroid adenoma.
Color Doppler US shows a prominent feeding artery entering the lesion from one pole (usually inferior pole) — the 'polar vessel sign'. This vessel is a branch of the inferior thyroid artery and is the vascular supply source of the adenoma. It branches within the lesion creating intranodular vascularity. The polar vessel sign is highly specific for parathyroid adenoma (80-90% specificity) and is distinguishing from thyroid nodule (peripheral vascularity) or lymph node (hilar vascularity).
Report Sentence
Doppler examination shows a prominent feeding artery entering from the [inferior/superior] pole of the lesion ('polar vessel sign'), a finding favoring parathyroid adenoma.
On Tc-99m sestamibi dual-phase scintigraphy, both thyroid and parathyroid adenoma show uptake in the early phase (15-20 min) — they overlap and may not be separable. In the delayed phase (2-3 hours), thyroid uptake washes out while persistent retention continues in the parathyroid adenoma — this 'delayed washout' is highly specific for parathyroid adenoma. Addition of SPECT/CT improves three-dimensional localization and increases detection of ectopic adenomas (mediastinum, retroesophageal). Sensitivity 80-90%, specificity 95+%. Sensitivity decreases in multiglandular disease (50-60%).
Report Sentence
Tc-99m sestamibi dual-phase scintigraphy shows a focal uptake area with persistent retention in the delayed phase posterior to the [right/left] thyroid lobe, consistent with parathyroid adenoma.
In 4D-CT (four-dimensional = three spatial dimensions + time/enhancement dimension) protocol, non-contrast, arterial, and delayed phase images are acquired. Parathyroid adenoma appears hypodense or isodense relative to thyroid parenchyma on non-contrast phase. Shows intense enhancement in arterial phase — becomes markedly hyperdense relative to thyroid parenchyma (adenoma's rich vascular structure). Shows washout in delayed phase, becoming hypodense again relative to thyroid. This triphasic enhancement dynamic (hypodense → hyperdense → hypodense) is highly specific for parathyroid adenoma. 4D-CT sensitivity ranges 85-95% and provides excellent anatomical detail for surgical planning.
Report Sentence
4D-CT shows a lesion measuring ... mm posterior to the [right/left] thyroid lobe, intensely enhancing in arterial phase with washout in delayed phase, consistent with parathyroid adenoma.
On T2-weighted MRI, parathyroid adenoma appears hyperintense relative to thyroid parenchyma. Increased water content of cellular adenomatous tissue prolongs T2 time. Shows intermediate-low signal on T1-weighted sequences. Marked enhancement on contrast-enhanced series. MRI is used especially in cases where US and scintigraphy are discordant, in ectopic adenomas, and in reoperation cases. On chemical shift MRI, parathyroid adenoma does not show opposed-phase signal drop unlike thyroid (no intracellular lipid accumulation in parathyroid tissue).
Report Sentence
MRI shows a T2 hyperintense, T1 hypointense, enhancing solid lesion posterior to the [right/left] thyroid lobe, consistent with parathyroid adenoma.
Criteria
Most common type (80-85%). Composed predominantly of chief cells. Usually <2 cm. Typical hypoechoic, homogeneous appearance on US.
Distinct Features
Moderate delayed sestamibi retention — less prominent than oxyphil adenoma. Homogeneous hypoechoic appearance on US. PTH level usually 2-3 times elevated.
Criteria
Composed of oxyphil cells — cells very rich in mitochondria. Less common (5-10%). Usually larger (>2 cm). Sestamibi uptake is very prominent.
Distinct Features
Very prominent delayed sestamibi retention (mitochondrial richness). May be more heterogeneous on US. More intense enhancement on 4D-CT. PTH level variable.
Criteria
Cystic degeneration or functional cyst formation within parathyroid adenoma. Prevalence 1-5%. Mixed cystic-solid lesion or completely cystic appearance on US.
Distinct Features
Can mimic thyroid simple cyst — posterior thyroid location and hypercalcemia/elevated PTH are distinguishing. High PTH in cyst aspirate is diagnostic. Low sestamibi uptake (scant solid component). US-guided aspiration + PTH measurement is diagnostic.
Criteria
Location outside normal anatomical position. Intrathyroid (1-3%), carotid sheath, intrathymic, aortopulmonary window, retroesophageal, or upper mediastinum. Results from embryological migration anomaly.
Distinct Features
May not be detected by US — sestamibi SPECT/CT and 4D-CT are superior for ectopic localization. Intrathyroid adenoma mimics thyroid nodule — high PTH in FNA aspirate is diagnostic. Sternotomy may be needed for mediastinal ectopic adenoma.
Distinguishing Feature
Thyroid nodule is intrathyroid (within thyroid capsule); parathyroid adenoma is extrathyroidal (posterior to capsule). Thyroid nodule shows peripheral vascularity (capsular ring); parathyroid adenoma shows polar vessel sign. Calcium and PTH levels are normal (elevated in parathyroid adenoma).
Distinguishing Feature
Cystic parathyroid adenoma can mimic thyroid simple cyst. Distinguishing: parathyroid adenoma is posterior to thyroid (outside capsule), high PTH in cyst aspirate (>100 pg/mL), hypercalcemia and elevated serum PTH. Simple cyst is intrathyroid, normocalcemic, and aspirate PTH is low.
Distinguishing Feature
Thyroid lymphoma usually appears as an intrathyroid hypoechoic mass with rapid growth. Parathyroid adenoma is extrathyroidal, smaller, and slower growing. Hypercalcemia and elevated PTH are not expected in lymphoma (except paraneoplastic hypercalcemia). Sestamibi is usually negative in lymphoma.
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralParathyroid adenoma treatment is surgery (parathyroidectomy), which is curative — with a 95-98% success rate. Minimally invasive parathyroidectomy (MIP) is the preferred approach for solitary adenomas accurately localized by preoperative US + sestamibi. Intraoperative PTH measurement (iPTH) confirms successful resection — a >50% PTH drop in 10 minutes is curative (Miami criterion). Surgical indications: symptomatic hyperparathyroidism (renal stones, osteoporosis, neuropsychiatric), asymptomatic but Ca >11.5 mg/dL or GFR <60 or T-score <-2.5 or age <50. Cinacalcet (calcimimetic) is a medical treatment option for patients who cannot undergo surgery. US-guided ethanol injection is an alternative in highly selected cases.
Parathyroid adenoma is the most common cause of primary hyperparathyroidism. Treatment is surgical excision (parathyroidectomy). Preoperative localization (US + sestamibi or 4D-CT) guides surgery. Intraoperative PTH monitoring confirms surgical success. Cure rate is >95%.