paratireoides

Paratireoides, or parathyroid glands, are small endocrine glands typically four in humans, located on the posterior surface of the thyroid gland in the neck. They vary in number and position, and ectopic glands can be found along the path of thymic descent or in the mediastinum.

Anatomy and development: Each gland is a few millimeters in size. Blood supply is usually via the inferior thyroid arteries, with venous drainage into the thyroid venous plexus. Embryologically, the paratireoides arise from the third and fourth pharyngeal pouches; the inferior glands commonly descend with the thymus, which explains some positional variation.

Histology: The glands are composed mainly of chief cells that secrete parathyroid hormone (PTH); oxyphil cells are present in smaller numbers and their function is not fully clear. PTH is the principal regulator of calcium homeostasis in humans.

Function and regulation: PTH raises serum calcium and lowers serum phosphate. It increases bone resorption of calcium and phosphate, enhances renal calcium reabsorption in the distal tubule, decreases phosphate reabsorption in the proximal tubule, and stimulates the production of active vitamin D (1,25-dihydroxyvitamin D) in the kidney, which boosts intestinal calcium absorption. Calcium-sensing receptors on parathyroid cells sense plasma calcium and modulate PTH secretion via negative feedback.

Clinical aspects: Disorders include primary hyperparathyroidism (usually from a adenoma or hyperplasia causing hypercalcemia), secondary hyperparathyroidism (often due to chronic kidney disease or vitamin D deficiency), and hypoparathyroidism (often after thyroid or parathyroid surgery) causing hypocalcemia with neuromuscular symptoms. Diagnosis and management rely on biochemical testing, imaging to localize glands, and, when indicated, surgical removal. Familial syndromes such as MEN1 can involve paratireoides.