Parathyroid

• Parathyroid glands are two pairs 
• superior and inferior
• small endocrine glands, that usually lie on the posterior border of the thyroid gland, within the false capsule. 
• The superior parathryroids are also referred to as parathyroid lV because they develop from the endoderm of the fourth pharyngeal pouch. 
• The inferior parathyroids, similarly, are also called parathyroid III because they develop from the third pouch.
• The parathyroids secrete the hormone parathormone
• which controls the metabolism of calcium and phosphorus along with thyrocalcitonin.
• Each parathyroid gland is oval or lentiform in shape,
• measuring 6 x 4 x 2 mm (the size of a split pea). 
• Each gland weighs about 50 mg.

Position

• The anastomosis between the superior and inferior thyroid arteries is usually a good guide to the glands because they usually lie close to it.
• The superior parathyroid is more constant in position and usually lies at the middle of the posterior border of the lobe of the thyroid gland. 
• It is usually dorsal to the recurrent laryngeal nerve.
• The inferior parathyroid is more variable in position.
• It may lie:
• a. Within the thyroid capsule, below the inferior thyroid artery and near the lower pole of the thyroid lobe.
• b. Behind and outside the thyroid capsule, immediately above the inferior thyroid artery.
• c. Within the substance of the lobe near its posterior border. 
• It is usually ventral to the recurrent laryngeal nerve.

Vascular Supply

• The parathyroid glands receive a rich blood supply from the inferior thyroid artery and from the anastomosis between the superior and inferior thyroid arteries. 
• The veins and lymphatics of the gland are associated with those of the thyroid and the thymus

Nerve Supply

• Vasomotor nerves are derived from the middle and superior cervical ganglia. 
• Parathyroid activity is controlled by blood calcium levels; low levels stimulate and high levels inhibit the activity of the glands.

CLINICAL ANATOMY

• Tumours of the parathyroid glands lead to excessive secretion of parathormone (hyperparathyroidism).
• This leads to increased removal of calcium from bone making them weak and liable to fracture. 
• Calcium levels in blood increase (hypercalcaemia) and increased urinary excretion of calcium can lead to the formation of stones in the urinary tract.
• Hypoparathyroidism may occur spontaneously or from accidental removal of the glands during thyroidectomy. 
• This results in hypocalcaemia leading to increased neuromuscular irritability causing muscular spasm and convulsions (tetany)

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Parathyoid Gland | Location |Development |Blood Supply | Microscopic structure | Clinical Importance