Parathyroid glands

The parathyroid glands are endocrine glands located in the visceral space of the neck. They produce parathyroid hormone, which controls calcium homeostasis. 

Gross anatomy

There are normally two pairs of parathryoid glands, inferior and superior, although there can be up to twelve in number. The parathyroid gland is oval or bean-shaped. It typically measures 6 x 4 x 2 mm and weighs 40-60 mg ^1-2. 

The parathyroid glands can have a variable location (from carotid bifurcation to anterior mediastinum) but are usually in close relation to the thyroid gland, in the region of the tracheo-oesophageal groove:

• superior parathyroid glands: located at the posterior aspect of the middle-third of the thyroid gland
• inferior parathyroid glands: located lateral to the inferior pole of the thyroid gland

The parathyroid glands are usually extracapsular to the thyroid but sometimes they may lie intracapsular. 

Blood supply

• arterial supply
  • superior parathyroid glands: supplied by the superior thyroid artery
  • inferior parathyroid glands: supplied by the inferior thyroid artery
• venous drainage: via thyroid veins

Variant anatomy

• supernumerary glands:incidence: incidence ~15% (range 3-25%) ^2-3
• fewer than four parathyroid glands: incidence <3% ²
• ectopic parathyroid gland: most commonly inferior to the inferior pole of the thyroid ¹

Radiographic features

Ultrasound and nuclear medicine are the predominant parathyroid imaging techniques with cross-sectional (CT/MRI) imaging usually reserved to confirm the anatomical position of ectopic thyroid adenomas ^2-3. 

Ultrasound

• high-frequency (5-10 MHz) probe
• homogeneously hypoechoic when compared to overlying echogenic thyroid gland

Nuclear medicine

• the most common radiotracer for parathyroid imaging is ^99mTc-sestamibi 

Development

• superior parathyroid glands derive from the fourth pharyngeal pouch
• inferior parathyroid glands derive from the third pharyngeal pouch

Related pathology

• hyperparathyroidism
• parathyroid adenoma