There are three calcium-regulating hormones in humans that play critical roles in regulating calcium levels in the body and maintaining healthy bones. These hormones are parathyroid hormone, calcitriol and calcitonin. Parathyroid hormone stimulates bone formation and resorption and maintains calcium levels in the body. Calcitriol, which is made from vitamin D, stimulates the intestines to absorb sufficient calcium and phosphorus and also has a direct effect on the bones. Calcitonin is responsible for inhibiting bone breakdown and protecting the body from abnormally high levels of calcium.
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The parathyroid glands are four small glands located near the thyroid in the neck. They produce one of the human calcium-regulating hormones, parathyroid hormone (PTH), which precisely controls blood calcium levels. The parathyroid glands are very sensitive to even small changes in the concentration of calcium in the blood and therefore play an important role in the processcalcium regulation. When a drop in blood calcium levels is detected, the parathyroid glands increase secretion of PTH. PTH tells the kidneys to conserve calcium. It also stimulates the production of another hormone, calcitriol, which increases calcium absorption in the gut. PTH also affects the bones and increases the transfer of calcium from the bones to the blood.
Excessive secretion of PTH, one of the human calcium-regulating hormones, is known as hyperparathyroidism. Excess PTH, for example from a secreting tumor of the parathyroid glands, can cause bone loss. In addition to bone resorption, PTH has been found to stimulate bone formation and strengthen bones when the hormone is administered in small amounts intermittently. The latest treatmentsosteoporosisare based on this feature of PTH.
Another hormone called PTH-related protein has recently been identified. Under normal circumstances, this hormone regulates bone and cartilage formation in the developing fetus. People with certain types of cancer develop a condition known as malignant hypercalcaemia, which is due to overproduction of a PTH-related protein, which acts like PTH and causes excessive bone breakdown and abnormally high levels of calcium in the blood.
Also known as 1,25-dihydroxy vitamin D, this is one of the human calcium-regulating hormones. It is a derivative of vitamin D. Enzymes in the kidneys and liver produce this hormone, which affects many different tissues and organs. The main function of calcitriol is to increase the absorption of calcium and phosphorus from the gut and to make these minerals available to the skeleton. Vitamin D is not an essential nutritional element and can be synthesized in the skin from cholesterol by exposure to ultraviolet light (sunlight); Therefore, it is not technically a vitamin. However, insufficient exposure to sunlight can lead to low levels of vitamin D, which must be supplemented from dietary sources.
The need for vitamin D in the diet has increased recently. Deficiency of this vitamin has become more common as people spend more and more time indoors and cover tightly when outdoors to avoid sunburn. People living in northern latitudes are particularly prone to vitamin D deficiency because in these parts of the world the sun's rays are not strong enough for the skin to produce enough vitamin D. Vitamin D is related to calcium-regulating hormones in humans.
A lack of vitamin D in the body leads to improper bone mineralization and a condition known as rickets or osteomalacia (in children and adults respectively). Rickets is associated with bone pain and characteristic bending of the legs and other bone deformities. Osteomalacia is associated with an increased susceptibility to fractures. Vitamin D supplementation can restore bone calcium and reduce bone loss.
In addition to parathyroid hormone and calcitriol, calcitonin is the third of the human calcium-regulating hormones. It is produced in the thyroid gland (by cells other than those that produce thyroid hormone). Calcitonin inactivates osteoclasts and prevents bone breakdown. Calcitonin plays an important role in bone development and calcium regulation at a young age. In adults, the effects of this hormone are temporary. Too high or too low a calcitonin level in an adult does not cause problems with blood calcium levels or bone strength. However, it is possible to use this hormone as a drug to treat bone diseases.
In addition to the three human calcium-regulating hormones (PTH, calcitriol, and calcitonin), sex hormones such as estrogen and testosterone play important roles in regulating skeletal growth and maintaining bone mass and strength. Estrogen and testosterone affect bone health in both men and women. The increased bone formation or the growth spurt during puberty is supported by sex hormones. Cartilaginous plates at the ends of long bones lengthen those bones, resulting in the dramatic increase in height normally seen during puberty. At the end of adolescence, the high concentration of sex hormones leads to the closure of the epiphyseal growth plates and the replacement of cartilage by bone at these sites. This prevents further growth after puberty.
Similar to human calcium-regulating hormones and their effects on the skeleton, estrogen stimulates new bone formation. It also acts on osteoclasts to inhibit bone loss at different stages of life. In women going through menopause, the level of estrogen in the body decreases significantly. For this reason, menopause is associated with a rapid loss of bone mass. Exogenous estrogen hormone replacement therapy (HRT) is given to women going through the menopause to prevent this age-related bone loss. However, the use of HRT is controversial because this therapy is associated with an increased risk of breast cancer, cardiovascular disease and blood clot formation.
Human calcium-regulating hormones affect bone through calcium regulation. Testosterone plays an important role in skeletal growth by acting directly on the bones. Testosterone also stimulates muscle growth, which puts pressure on the bones and thus encourages bone formation. Fat cells convert testosterone to estrogen, allowing testosterone to serve as a source of estrogen in the body. Estrogen is important for bone health in both men and women. In fact, levels of the circulating hormone, estrogen, are higher in older men than in older (postmenopausal) women.
Other hormones involved in bone health
In addition to the three human calcium-regulating hormones and the sex hormones, there are other hormones that also play an important role in regulating skeletal growth. One of these hormones is growth hormone, which is produced by the pituitary gland in the brain. Growth hormone stimulates the liver to produce and release large amounts of insulin-like growth factor, or IGF-1, into the circulation. Besides the liver, IGF-1 hormone is also produced locally in bone and some other tissues, also under the regulation of growth hormone. In addition, growth hormone also has a direct effect on bones (apart from IGF-1).
The skeletal growth observed during puberty is accelerated by growth hormones. Inold people, growth hormone, and IGF-1 production decrease, so with age, the ability to replace resorbed bone or form new bone rapidly decreases. Although growth hormone and IGF-1 stimulate both bone formation and resorption, they act primarily on bone-forming cells with a consequent increase in bone mass.
In addition to the calcium-regulating hormones in humans, the hormones produced by the thyroid affect bones by increasing the energy produced by all cells in the body, including bone cells. This increased energy production leads to increased bone formation as well as bone resorption. A lack of thyroid hormones in childhood leads to growth disorders. Abnormally high levels of thyroid hormones in the blood can lead to excessive bone loss and a weak skeleton. In addition, it is believed that TSH, or thyrotropin, a hormone produced by the pituitary gland that regulates the production of thyroid hormones, may also directly affect bones.
Cortisol, a hormone produced by the adrenal glands, is vital in regulating metabolism in the body. It plays a crucial role in the body's ability to cope with injury and stress. The effects of cortisol on the skeleton are complex. While small amounts of cortisol are necessary for normal bone development, when present in large amounts, cortisol blocks bone growth. Along with the calcium-regulating hormones in humans, it is one of the hormones that affect the bones.
Glucocorticoids (synthetic cortisol) are used to treat conditions such as arthritis and asthma. Long-term use of these drugs can cause bone loss due to increased bone breakdown and decreased bone formation. Therefore, prolonged treatment with high doses of glucocorticoids can put a person at high risk of fracture.
In addition to the calcium-regulating hormones mentioned above in humans, a number of other hormones circulating in the bloodstream also have an effect on the skeleton. Insulin plays an important role in bone growth. In people with insulin deficiency, there is an altered response to other factors that stimulate bone growth. Leptin, a recently discovered hormone made by fat cells to regulate energy levels, is also thought to affect the skeleton.
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