Calcium is the most abundant mineral in the body and 99 percent is contained within the skeleton. Bones act as a reservoir for calcium and only five percent of the calcium in the bone is turned over each day during bone remodeling. The actions of the nervous system and muscles are dependent on calcium levels in the blood and proper functioning requires tight control of calcium levels. Colorado State University in the “Pathophysiology of the Endocrine System” states the three organs that are involved in maintaining calcium levels in the body are the bones, kidney and small intestine. Parathyroid hormone, vitamin D and calcitonin are the hormones that maintain calcium homeostasis by stimulating certain actions of these organs.
Parathyroid Hormone
There are four parathyroid glands, each the size of a rice grain, located on the back side of the thyroid gland in the neck.
Cells in the parathyroid glands directly respond to calcium levels in the blood. If calcium levels are low the cells increase parathyroid hormone release. Parathyroid hormone increases blood calcium levels by activating bone resorption, stimulating the kidneys to increase calcium reabsorption and upregulating enzymes in the kidney that convert vitamin D to its activated form, which then increases calcium uptake in the intestine. Parathyroid hormone is the primary hormone involved in regulating calcium levels.
Vitamin D
Vitamin D is converted to the active hormone, calcitriol whichl is delivered in the blood stream to its target cells, primarily in the small intestine. Calcitriol moves to the nucleus of these cells and increases the expression of proteins that transport calcium from the inside the small intestine to the blood.
Calcitonin
Calcitonin, secreted by C-cells located in the thyroid gland, is the opposing hormone of parathyroid hormone. Calcitonin decreases blood calcium levels by inhibiting bone resorption, increasing calcium loss to the urine and by decreasing intestinal calcium absorption. Though the primary regulator of calcium homeostasis is parathyroid hormone. Many studies suggest calcitonin plays a very minor role in maintaining calcium levels. An article in the journal “Endocrine,” notes that neither a deficiency nor an excess in calcitonin causes disease and it is likely that calcitonin plays no role in calcium homeostasis except for maybe during calcium stress. In fact, the primary negative regulator of calcium levels in the body is calcium itself. A review article in “Current Medicinal Chemistry” reports that calcium receptors on cells of the parathyroid gland allow for the direct action of blood calcium levels on the expression and release of parathyroid hormone.
