Parathyroid Gland

The parathyroid glands are four very small, pea-sized (or smaller) endocrine glands, typically two embedded in the posterior surface of each of the thyroid gland's two lobes — an upper and a lower parathyroid gland on each side — though their exact number and precise position can vary somewhat between individuals. Despite lying physically within or immediately behind the thyroid tissue, the parathyroid glands are entirely distinct from the thyroid both in their embryonic origin and in the hormone they secrete, and it is important not to confuse the two simply because of their shared location in the neck.

The parathyroid glands secrete a single hormone, parathyroid hormone (PTH), also called parathormone, whose overall effect is to raise blood calcium level whenever it falls below the normal physiological range — for this reason PTH is described as a hypercalcemic hormone. PTH achieves this rise in blood calcium through three coordinated actions on three different target tissues. In bone, PTH stimulates the activity of osteoclasts, the bone cells responsible for breaking down bone matrix, releasing the calcium stored within it into the blood. In the kidney, PTH increases the reabsorption of calcium at the distal convoluted tubule, reducing the amount of calcium lost from the body in urine. Also in the kidney, PTH stimulates the enzyme that converts vitamin D to its active hormonal form, calcitriol, which in turn acts on the small intestine to increase the absorption of dietary calcium into the blood. Through this combination of bone resorption, reduced renal calcium loss, and increased intestinal calcium absorption, PTH secretion — triggered directly by a fall in blood calcium level, sensed by the parathyroid cells themselves — restores blood calcium toward its normal range. …