b) Parathyroid Glands

The parathyroid glands are involved in the production of parathyroid hormone (PTH), a small peptide that is involved in calcium metabolism. The glands are renowned for being highly variable in number and position.

Structure

The most common description of parathyroid glands is of small, 6 mm long oval structures that are only about 1 mm thick. They are usually found posterior to the thyroid gland, between the glandular substance and the capsule. They do not form lobes and are usually yellowish in colour.

Variations

There may be fewer or greater numbers of parathyroid glands than the usual four. Glands may be located outside of the vicinity of the thyroid, sometimes as high as the carotid bifurcation or within the thymus gland. Some cases of occult parathyroid tissue occur where there are multiple microscopic deposits of glandular tissue.

Microscopic Structure

The parathyroid glands consist of chief cells which secrete PTH surrounded by a fine capillary network.

Relations

Relations are highly dependent on the position of the glands; the most usual pattern is for the thyroid to be anterior and the carotid and jugular vessels posterior. The trachea and oesophagus are medial.

Neurovascular Supply

Arterial Supply

Both superior and inferior parathyroid glands are usually supplied by branches of the inferior thyroid artery.

Venous Drainage

Venous blood drains to the plexus of anterior thyroid veins, and then returns through the superior, middle or inferior thyroid veins.

Lymphatics

Lymphatics accompany those of the thyroid gland, and may pass to anterior cervical nodes (station VI), inferior deep cervical nodes (station IV) or to anterior mediastinal nodes.

Innervation

The vessels of the parathyroid glands receive sympathetic inputs from the superior and middle cervical ganglion. There is no known secrotomotor innervation.

Potential Routes of Malignant Spread

Local invasion may involve the thyroid gland, trachea or oesophagus leading to symptoms. Lymphatic spread can be bilateral and extensive.


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