Inferior parathyroid gland

At a glance

English

Inferior parathyroid gland

Latin (TA2)

Glandula parathyreoidea inferior

System

Endocrine gland

Region

Whole body

Side

Paired, left and right

About the inferior parathyroid gland

Most people have four parathyroid glands, two on each side, pressed against the back of the thyroid gland. The inferior pair are the lower ones. Each is a flat, oval, yellowish-brown body about the size of a grain of rice, roughly six millimetres long, and sits within the thyroid's fibrous sheath or just outside it, near the lower pole of the lateral lobe. A typical site is close to where the inferior thyroid artery enters the gland, below and in front of the recurrent laryngeal nerve.

Their position is much less constant than that of the superior glands, and the reason is developmental. The inferior glands arise from the third pharyngeal pouch together with the thymus, and are dragged downwards as the thymus descends into the chest. They therefore end up below the superior glands, even though they come from a higher pouch, and may stop anywhere along this path: high in the neck, low beside the trachea, or within the thymus in the upper mediastinum.

Microscopically each gland contains cords of chief cells, which make parathyroid hormone, together with oxyphil cells that become more numerous with age, and fat cells. Parathyroid hormone raises the calcium level of the blood by releasing calcium from bone, increasing its reabsorption in the kidney and stimulating the kidney to activate vitamin D, which boosts absorption from the gut. It also promotes the loss of phosphate in the urine.

Clinically the glands are at risk during thyroid surgery, when they may be removed or lose their blood supply, leading to low blood calcium with tingling and muscle spasms. An overactive gland, most often a single benign adenoma, causes high calcium, kidney stones and bone thinning. A missing inferior gland may need to be sought in the thymus.

Function and supply

Function

Secretes parathyroid hormone, which raises blood calcium and lowers blood phosphate by acting on bone, kidney and, through vitamin D, the gut.

Nerve supply

Sympathetic fibres from the cervical ganglia reach it along its vessels; secretion is controlled by the blood calcium level, not by nerves.

Blood supply

Mainly the inferior thyroid artery; venous blood drains into the thyroid venous plexus.