The parathyroid gland in humans is a 4 glands that are situated in the posterior or behind the thyroid gland. There is a capsule of connective tissue that separates the parathyroid gland from the thyroid gland.
The parathyroid gland is supplied with blood from the inferior thyroid artery or one of its branches.
The parathyroid gland is important for living so that without it, life cannot happen. Usually thyroidectomy removes also part of the parathyroid glands as well. Removal of all the parathyroid glands causes death because of the inability of the body to produce calcium. This condition causes so called tetany or continuous contraction or spasm of the muscles of the body especially the muscles of the larynx which its contraction causes suffocation and death.
The parathyroid glands contain two types of cells. These are the chief cells and the oxyphil cells. The chief cells are the site of production and secretion of the parathyroid hormone. The oxyphil cells have no apparent role in parathyroid hormone secretion.
Parathyroid hormone is an antagonist to the hormone calcitonin. While calcitonin functions by causing deposition of calcium phosphate on bones, the parathyroid hormone functions as an antagonist by increasing resorption of calcium phosphate from bones to the extracellular matrix.
In addition to causing resorption of bones to increase the blood level of calcium and phosphate ions, the parathyroid hormone functions by increasing the kidney conservation of calcium. It does so in the tubules of the nephrons where most filtered calcium is reabsorbed, while phosphate is secreted in the urine.
Parathyroid hormone increases also the the reabsorption of magnesium and hydrogen ions, whie it decreases the reabsorption of sodium and potassium and amino acids.
In addition to these two important roles of the parathyroid hormone on the resorption of calcium phosphate and its stimulation of the kidney to reabsorp calcium it increases also the absorption of calcium from the intestinal tract. It does this indirectly by increasing the synthesis of the active form of vitamin D (1,25, dihydrocholecalciferol). 1,25,dihydrocholecalciferol functions by increasing the absorption of calcium from the intestine.
Hyperparathyroidism or excessive secretion of the parathyroid hormone occurs often in women during pregnancy and during lactation, in which the parathyroid gland becomes enlarged due to the diminished level of calcium in the blood. Stones in the kidney can occur as a result of excess calcium in the blood due to the hyperparathyroidism.
Hypoparathyroidism can be caused due to increased level of calcium or vitamin D in the blood due to high intake in the diet or it can happen also due to surgery in which part of the parathyroid gland is removed.