parathyroids

The parathyroids are four small endocrine glands usually located on the posterior surface of the thyroid gland. They are typically arranged as two superior and two inferior glands, but their number and exact position can vary; ectopic parathyroids may be found along the course of the thymic descent or in the mediastinum, retroesophageal space, or along the carotid sheath.

The primary function of the parathyroids is to secrete parathyroid hormone (PTH), a key regulator of calcium and phosphate homeostasis. PTH elevates serum calcium by stimulating osteoclast-mediated bone resorption, increasing renal calcium reabsorption in the distal nephron, and promoting production of active vitamin D (1,25-dihydroxyvitamin D3) by the kidneys, which enhances intestinal calcium absorption. PTH also reduces renal phosphate reabsorption, lowering serum phosphate.

Secretion of PTH is tightly controlled by calcium-sensing receptors on chief cells; low serum calcium stimulates PTH release, whereas high calcium suppresses it. Magnesium status and vitamin D levels modulate secretion and action as well.

Embryologically, the parathyroids arise from the third (inferior) and fourth (superior) pharyngeal pouches. Histologically, the gland is composed predominantly of chief (principal) cells that secrete PTH, with smaller numbers of oxyphil cells; the glands are embedded in a capsule and may be interspersed with fat, especially with aging.

Disorders of the parathyroids include primary hyperparathyroidism (usually from a solitary adenoma or hyperplasia), secondary hyperparathyroidism (often due to chronic kidney disease), and tertiary hyperparathyroidism (autonomous PTH secretion after prolonged secondary hyperparathyroidism). Hypoparathyroidism, most commonly after neck surgery or autoimmune disease, causes hypocalcemia. Diagnosis relies on serum calcium, phosphate, and PTH levels; imaging (ultrasound, sestamibi scintigraphy) may aid surgical localization. Treatment ranges from medical management to surgical removal (parathyroidectomy) of diseased tissue.