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Structure

Parathyroid glands

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The parathyroid glands are small endocrine glands on the posterior surface of the thyroid that secrete parathyroid hormone (PTH), the main regulator of serum calcium. There are usually four, but the number and position vary.

Terminologia Anatomica (Latin): Glandulae parathyroideae · Open in the 3D atlas

Location

Two superior and two inferior glands usually lie on the posterior border of the thyroid lobes, within or just outside the thyroid capsule. The inferior glands are more variable in position (they may be intrathymic or descend into the mediastinum) because they develop from the third pharyngeal pouch with the thymus; the superior glands arise from the fourth pouch.

Structure

Each gland is about the size of a grain of rice (approximately 5 x 3 x 1 mm, total weight about 120-150 mg in adults, variable), yellow-brown, and composed of chief cells that secrete PTH and larger oxyphil cells, with increasing fat in the stroma with age. Supernumerary glands occur in a minority.

Blood supply

Both glands on each side are mainly supplied by the inferior thyroid artery, with the superior glands also receiving anastomotic branches from the superior thyroid artery. Venous drainage is to the thyroid veins, and lymph follows the thyroid to the deep cervical and paratracheal nodes.

Innervation

Sympathetic fibres from the cervical ganglia regulate vascular tone, but PTH secretion is controlled directly by the plasma ionised calcium level through the calcium-sensing receptor, not by nerves.

Function

PTH raises serum calcium by increasing bone resorption, renal calcium reabsorption and renal calcitriol synthesis (hence intestinal calcium absorption), while lowering renal phosphate reabsorption. Secretion rises when ionised calcium falls.

Clinical correlations

Primary hyperparathyroidism (usually a single adenoma) causes hypercalcaemia with stones, bones, abdominal groans and psychic moans; hypoparathyroidism, most often after thyroid or parathyroid surgery, causes hypocalcaemia with tetany, perioral paraesthesia and a prolonged QT interval. Secondary hyperparathyroidism occurs in chronic kidney disease. Management follows local guidelines.

Common exam points

  • Usually four glands (variable); superior from the 4th pouch, inferior from the 3rd pouch.
  • Inferior glands are the more variable in position.
  • Supplied mainly by the inferior thyroid artery.
  • PTH: raises calcium, lowers phosphate, stimulates calcitriol synthesis.
  • Post-thyroidectomy hypocalcaemia: tetany, Chvostek and Trousseau signs.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Williams Textbook of Endocrinology. Elsevier, 14th edition (2019) — Tier 3 (textbook)

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