paratyroid

Parathyroid glands, also spelled paratyroid in some texts, are small endocrine glands typically located on the posterior surface of the thyroid gland in the neck. Most individuals have four glands—two superior and two inferior—but the number and position can vary, with ectopic or supernumerary glands described in some people.

They secrete parathyroid hormone (PTH), which regulates calcium and phosphate homeostasis. PTH raises serum calcium by stimulating bone resorption, increasing renal calcium reabsorption, and promoting activation of vitamin D in the kidney, which increases intestinal calcium absorption. It lowers serum phosphate by reducing renal phosphate reabsorption. The net effect of PTH is to maintain adequate calcium for neuromuscular and other cellular functions.

Regulation: PTH secretion is controlled by calcium-sensing receptors in parathyroid chief cells; low serum calcium stimulates PTH release, while high calcium suppresses it. Vitamin D feedback also influences PTH levels.

Clinical relevance: Primary hyperparathyroidism is the most common disease of the glands, usually due to a benign adenoma, hyperplasia, or rarely carcinoma, leading to hypercalcemia, bone loss, kidney stones. Secondary hyperparathyroidism results from chronic hypocalcemia (often due to CKD or vitamin D deficiency), causing diffuse gland enlargement and excessive PTH. Tertiary hyperparathyroidism occurs after long-standing secondary hyperparathyroidism, with autonomous PTH secretion. Hypoparathyroidism, often due to surgical removal or autoimmune destruction, causes hypocalcemia with neuromuscular irritability and other symptoms.

Embryology and blood supply: The glands arise from the third and fourth pharyngeal pouches; most receive arterial supply from branches of the inferior thyroid artery; venous drainage via thyroid veins. They are commonly removed or preserved during thyroid or neck surgery.