There are normally two pairs of parathyroid 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 .
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-esophageal groove:
- superior parathyroid glands: located at the posterior aspect of the middle third of the thyroid gland, posterior to the inferior thyroid artery
- inferior parathyroid glands: located lateral to the inferior pole of the thyroid gland, anterior to the inferior thyroid artery
The parathyroid glands are usually extracapsular to the thyroid but sometimes they may lie intracapsular.
- arterial supply
- venous drainage: via superior, middle and inferior thyroid veins
Lymphatic vessels drain into the pretracheal, prelaryngeal and jugulodigastric lymph nodes.
Sympathetic vasoconstrictor fibers from the middle and inferior cervical ganglia enter with the arteries.
- supernumerary glands: incidence ~15% (range 3-25%)
- fewer than four parathyroid glands: incidence <3%
- ectopic parathyroid gland: most commonly inferior to the inferior pole of the thyroid
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 parathyroid adenomas .
- high-frequency (5-10 MHz) probe
- homogeneously hypoechoic as compared to the echogenic thyroid
- the most common radiotracer for parathyroid imaging is Tc-99m sestamibi (taken up by abnormal parathyroid (high mitochondria in adenomas and hyperplasia) and thyroid)
- normal parathyroid glands do not generally take up sestamibi
- Tc-99m pertechnetate is taken up by thyroid only (pertechnetate mimics iodine molecule for the Na/I transporter)
- superior parathyroid glands derive from the fourth pharyngeal pouch
- often located posterior to inferior thyroid artery
- inferior parathyroid glands derive from the third pharyngeal pouch
- the thymus is also derived from the third pouch - the inferior parathyroids are often located between the fascial plane between the thyroid gland and thymus - i.e. anterior to inferior thyroid artery)
- see branchial apparatus