Paratyroidea

Paratyroidea, commonly called the parathyroid glands, are small endocrine glands typically located on the posterior surface of the thyroid gland in mammals. In humans there are usually four glands—two superior and two inferior—but variations such as supernumerary or ectopic glands can occur. They originate from the third and fourth pharyngeal pouches during embryonic development.

The glands are composed mainly of chief (principal) cells, which secrete parathyroid hormone (PTH), and smaller oxyphil cells. PTH is the primary regulator of calcium and phosphate homeostasis. It raises serum calcium by increasing bone resorption of calcium through osteoblast–osteoclast signaling, enhancing renal calcium reabsorption, reducing renal phosphate reabsorption, and promoting activation of vitamin D in the kidney, which in turn increases intestinal calcium absorption.

PTH secretion is tightly regulated by circulating calcium levels: low calcium stimulates PTH release, while high calcium suppresses it. Imaging and localization techniques, such as ultrasound and sestamibi scintigraphy, are used when surgical intervention is planned.

Clinical significance includes disorders of calcium balance. Primary hyperparathyroidism, usually due to a parathyroid adenoma, hyperplasia, or rarely carcinoma, causes hypercalcemia with symptoms such as bone pain, kidney stones, and fatigue. Secondary hyperparathyroidism arises from chronic kidney disease or severe vitamin D deficiency, leading to enlarged glands and elevated PTH. Hypoparathyroidism, often following surgical removal or autoimmune destruction, results in hypocalcemia and neuromuscular irritability. Management ranges from monitoring in mild cases to parathyroidectomy for surgical disorders, with pharmacologic options such as calcimimetics and vitamin D analogs as needed.