The parathyroid glands are small, pea-sized endocrine glands located on the rear side of the thyroid gland. The major function of the parathyroid glands is to maintain the body's calcium level within a very narrow range, so that the nervous and muscular systems, which depend on calcium to transmit action potentials, can function properly.
When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland are activated, and the parathyroid glands release parathyroid hormone into the blood. Parathyroid hormone is a small protein hormone that is integral to the regulation of the level of calcium in the blood via the bone, kidneys, and intestines . Parathyroid hormone works in concert with another hormone, calcitonin, which is produced by the thyroid. Parathyroid hormone acts to increase blood calcium levels, while calcitonin acts to decrease blood calcium levels. This interaction between parathyroid hormone and calcitonin is also an important part of bone remodeling. This is a lifelong process where mature bone tissue is removed from the skeleton and new bone tissue is formed.
Parathyroid hormone acts on the bone to increase blood calcium levels by stimulating osteoclasts to break down bone, releasing calcium into the bloodstream. Parathyroid hormone acts on the gastrointestinal tract to increase blood calcium by increasing the activity of the enzyme in the intestines that activates vitamin D. It acts on the kidneys to increase blood calcium levels by promoting calcium reabsorption in the nephrons. Parathyroid hormone also has an affect on the perception of well being. An absence of parathyroid hormone can be associated with feeling of fatigue and anxiety.