Parathyroid glands
Glandulae parathyroideae
Also known as: parathyroids
Usually four lentil-sized glands (≈30–50 mg each) on the posterior surface of the thyroid whose chief cells secrete parathyroid hormone, the minute-to-minute regulator of blood calcium.
Overview
Most people have four parathyroid glands (a superior and an inferior pair), though 3–6 occur and supernumerary glands are found in ≈5–10 %. Each is a flattened, oval, yellowish-brown body about 6 × 4 × 2 mm, lying on the posterior surface of the thyroid lobes, usually between the thyroid's true capsule and its fascial sheath.
The superior parathyroids (from the 4th pharyngeal pouch) are fairly constant, near the middle of the posterior border of each lobe at the level of the cricoid cartilage, above the inferior thyroid artery. The inferior parathyroids (from the 3rd pouch, which also forms the thymus) migrate further and are more variable — usually near the lower pole, below the inferior thyroid artery, but they can be in the thyrothymic ligament, within the thymus or in the superior mediastinum, which matters when searching for an adenoma.
Chief (principal) cells sense ionised calcium through the calcium-sensing receptor (CaSR), a Gq/Gi-coupled GPCR: low Ca²⁺ reduces CaSR signalling and releases PTH within seconds to minutes; sustained hypocalcaemia increases PTH gene expression and causes hyperplasia. Magnesium is required for PTH secretion.
- Location
Posterior surface of the thyroid lobes: superior pair at the level of the cricoid cartilage; inferior pair near the inferior poles (variable, sometimes mediastinal).
Anatomy
- Arterial supply
Mainly the inferior thyroid arteries (both pairs), with contributions from the superior thyroid arteries and anastomoses; preserving these end-arteries during thyroidectomy prevents hypoparathyroidism.
- Venous drainage
Into the thyroid venous plexus (superior, middle and inferior thyroid veins).
- Lymphatic drainage
With the thyroid to deep cervical and paratracheal nodes.
- Histology
Chief cells (small, pale-to-eosinophilic, PTH-secreting) and oxyphil cells (larger, intensely eosinophilic, packed with mitochondria; appear at puberty and increase with age; function uncertain); adipocytes increase with age (up to ≈50 % of the gland in the elderly).
- Development
Inferior glands from the dorsal wing of the 3rd pharyngeal pouch; superior glands from the 4th pouch.
Function
- Secrete parathyroid hormone (PTH) in response to falling ionised calcium.
- Thereby raise plasma calcium and lower phosphate (via bone, kidney and, indirectly, gut).
- Regulation
Ionised Ca²⁺ via CaSR (main); calcitriol inhibits PTH gene transcription; high phosphate and FGF-23 also modulate; severe hypomagnesaemia impairs PTH release (refractory hypocalcaemia).
Clinical significance
Primary hyperparathyroidism
Usually a single adenoma (≈80–85 %); hypercalcaemia with high/inappropriately normal PTH and low phosphate. Commonly asymptomatic; otherwise "stones, bones, groans, thrones and psychiatric overtones". Localise with sestamibi scan and ultrasound; cure by parathyroidectomy.
Hypoparathyroidism
Most often after thyroid/neck surgery; also autoimmune and DiGeorge syndrome. Hypocalcaemia causes perioral and digital paraesthesia, muscle cramps, tetany (carpopedal spasm), Chvostek sign (facial twitch on tapping the facial nerve) and Trousseau sign (carpal spasm with a BP cuff), seizures, prolonged QT.
DiGeorge syndrome (22q11.2 deletion)
Failure of 3rd and 4th pouch development: hypocalcaemia (absent parathyroids), thymic hypoplasia with T-cell deficiency, conotruncal heart defects, cleft palate and facial anomalies ("CATCH-22").
Test yourself
The inferior parathyroid glands develop from which pharyngeal pouch, explaining their variable (sometimes mediastinal) position?
- First
- Second
- Third
- Fourth
Answer: Third
The 3rd pouch gives the inferior parathyroids and the thymus; they migrate caudally together, so inferior glands can end up anywhere down to the anterior mediastinum. The 4th pouch gives the superior parathyroids, which migrate less.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.