The parathyroid glands (latin: glandula parathyroidea) are multiple small endocrine organs in the neck and anatomically closely associated with the thyroid gland. The parathyroid glands are essential in regulating calcium homeostasis.
Humans typically have four parathyroid glands, which are yellowish brown, ovoid, a few millimeters in diameter (6 mm long and 3 - 4 mm wide) and arranged in two pairs, constituting the superior and inferior parathyroid glands. Usually, the parathyroid glands are positioned within the connective tissue on the posterior surface of the left and right lobes of the thyroid gland, but their number and location may vary.
The parathyroid glands receive their blood supply from branches of the inferior thyroid arteries, which arise from the subclavian arteries. They can also be supplied by anastomoses between the superior and inferior thyroid arteries, the thyroid ima artery and other vessels of the collateral circulation. The parathyroid veins drain into the thyroid venous plexus.
The endocrine secretion of the parathyroid glands is regulated hormonally, while their blood vessels receive rich innervation from the sympathetic nervous system via branches arising from the thyroid branches of the cervical ganglia of the sympathetic chain.
The main function of the parathyroid glands is to hormonally regulate calcium levels within the blood. The parathyroid glands do this by producing the parathyroid hormone (PTH) that elevates the calcium levels through several mechanisms. Firstly, the PTH acts on cells of the bones causing them to release calcium into the bloodstream, thus also regulating the amount of calcium in the bones and therefore affecting the strength and density of the bones. Another way that the PTH can raise calcium levels is by acting upon the epithelial cells lining the intestinal tract to increase the absorption of calcium from the diet.